In linguistics, grammatical mood (also mode) is a grammatical feature of verbs, used for signaling modality.:p.181; That is, it is the use of verbal inflections that allow speakers to express their attitude toward what they are saying (e.g. a statement of fact, of desire, of command, etc.). The term is also used more broadly to describe the syntactic expression of modality - that is, the use of verb phrases that do not involve inflection of the verb itself.
Mood is distinct from grammatical tense or grammatical aspect, although the same word patterns are used for expressing more than one of these meanings at the same time in many languages, including English and most other modern Indo-European languages. (See tense-aspect-mood for a discussion of this.)
Some examples of moods are indicative, interrogative, imperative, subjunctive, injunctive, optative, and potential. These are all finite forms of the verb. Infinitives, gerunds, and participles, which are non-finite forms of the verb, are not considered to be examples of moods.
Some Uralic Samoyedic languages have more than ten moods; Nenets has as many as sixteen. The original Indo-European inventory of moods consisted of indicative, subjunctive, optative, and imperative. Not every Indo-European language has all of these moods, but the most conservative ones such as Avestan, Ancient Greek, and Sanskrit have them all. English has indicative, imperative, and subjunctive moods; other moods, such as the conditional, do not appear as morphologically distinct forms.
Not all of the moods listed below are clearly conceptually distinct. Individual terminology varies from language to language, and the coverage of (e.g.) the "conditional" mood in one language may largely overlap with that of the "hypothetical" or "potential" mood in another. Even when two different moods exist in the same language, their respective usages may blur, or may be defined by syntactic rather than semantic criteria. For example, the subjunctive and optative moods in Ancient Greek alternate syntactically in many subordinate clauses, depending on the tense of the main verb. The usage of the indicative, subjunctive, and jussive moods in Classical Arabic is almost completely controlled by syntactic context. The only possible alternation in the same context is between indicative and jussive following the negative particle l?.
Realis moods are a category of grammatical moods that indicate that something is actually the case or actually not the case. The most common realis mood is the indicative mood. Some languages have a distinct generic mood for expressing general truths. For other realis moods, see the main Realis mood article.
The indicative mood, or evidential mood, is used for factual statements and positive beliefs. It is the mood of reality. The indicative mood is the most commonly used mood and is found in all languages. Example: "Paul is eating an apple" or "John eats apples". All intentions that a particular language does not categorize as another mood are classified as indicative.
Irrealis moods are the set of grammatical moods that indicate that something is not actually the case or a certain situation or action is not known to have happened. They are any verb or sentence mood that is not a realis mood. They may be part of expressions of necessity, possibility, requirement, wish or desire, fear, or as part of counterfactual reasonings, etc.
Irrealis verb forms are used when speaking of an event which has not happened, is not likely to happen, or is otherwise far removed from the real course of events. For example, in the sentence "If you had done your homework, you wouldn't have failed the class", had done is an irrealis verb form.
Some languages have distinct irrealis grammatical verb forms. Many Indo-European languages preserve a subjunctive mood. Some also preserve an optative mood that describes events that are wished for or hoped for but not factual.
Common irrealis moods are the imperative, the conditional, the subjunctive, the optative, the jussive, and the potential. For other examples, see the main article for each respective mood.
The subjunctive mood, sometimes called conjunctive mood, has several uses in dependent clauses. Examples include discussing imaginary or hypothetical events and situations, expressing opinions or emotions, or making polite requests (the exact scope is language-specific). A subjunctive mood exists in English, though it is used in English much less than in many other Indo-European languages. In English, this mood has, for some uses, become something of a linguistic fossil. An example of the subjunctive mood is "I suggest that Paul eat an apple". The sentence refers to an event which may or may not take place. Contrast this with the indicative verb of the sentence "Paul will eat an apple", in which the verb "will eat" states an unambiguous fact. Another way of expressing the suggestion is "I suggest that Paul should eat an apple".
Other uses of the subjunctive in English are archaisms, as in "And if he be not able to bring a lamb, then he shall bring for his trespass..." (KJV Leviticus 5:7). Statements such as "I will ensure that he leave immediately" often sound archaic or overly formal, and have been almost completely supplanted by constructions with the indicative, like "I will ensure that he leaves immediately".
Some Germanic languages distinguish between two types of subjunctive moods, for example, the Konjunktiv I and II in German or the present and past subjunctive in English. Note that the latter distinction is not about the actual time at which something happens (or does not happen).
The conditional version of "John eats if he is hungry" is (subjunctive part boldfaced):
The subjunctive mood figures prominently in the grammar of the Romance languages, which require this mood for certain types of dependent clauses. This point commonly causes difficulty for English speakers learning these languages.
In certain other languages, the dubitative or the conditional moods may be employed instead of the subjunctive in referring to doubtful or unlikely events (see the main article).
The conditional mood is used for speaking of an event whose realization is dependent upon another condition, particularly, but not exclusively, in conditional sentences. In Modern English, this type of modality is expressed via a periphrastic construction, with the form would + infinitive, (e.g. I would buy), and thus is a mood only in the broad sense and not in the more common narrow sense of the term "mood" requiring morphological changes in the verb. In other languages, verbs have a specific conditional inflection. In German, the conditional mood is identical to one of the two subjunctive moods (Konjunktiv II, see above).
The conditional version of "John eats if he is hungry" is (conditional part boldfaced):
In the Romance languages, the conditional form is used primarily in the apodosis (main clause) of conditional sentences, and in a few set phrases where it expresses courtesy or doubt. The main verb in the protasis (dependent clause) is usually in the subjunctive or in the indicative mood. However, this is not a universal trait: among others in German (as above), Finnish and Romanian (even though the last is a Romance language), the conditional mood is used in both the apodosis and the protasis. A further example is the sentence "I would buy a house if I earned a lot of money", where in Finnish both clauses have the conditional marker -isi-: Ostaisin talon, jos ansaitsisin paljon rahaa. In Polish (as well as in eastern Slavic languages) the conditional marker -by also appears twice: Kupi?bym dom, gdybym zarabia? du?o pieni?dzy.
Because English is used as a lingua franca, a common error among second-language speakers is to use "would" in both clauses, e.g. *"I would buy if I would earn...". "Would" can, however, correctly be used after "if" in sentences such as "If you would only tell me what is troubling you, I might be able to help" (i.e. "if you were willing to tell me...").
The optative mood expresses hopes, wishes or commands and has other uses that may overlap with the subjunctive mood. Few languages have an optative as a distinct mood; some that do are Albanian, Ancient Greek, Kazakh, Japanese, Finnish, Nepali, and Sanskrit.
The imperative mood expresses direct commands, prohibitions, and requests. In many circumstances, using the imperative mood may sound blunt or even rude, so it is often used with care. Example: "Paul, do your homework now". An imperative is used for telling someone to do something without argument. Many languages, including English, use the bare verb stem to form the imperative (such as "go", "run", "do"). Other languages, such as Seri and Latin, however, use special imperative forms. In English, the second person is implied by the imperative except when first-person plural is specified, as in "Let's go" ("Let us go"). In Romance languages a first person plural exists in the imperative mood: Spanish: Vayamos a la playa; French: Allons à la plage (both meaning: Let us go to the beach). The prohibitive mood, the negative imperative may be grammatically or morphologically different from the imperative mood in some languages. It indicates that the action of the verb is not permitted, e.g. "Don't you go!" In English, the imperative is sometimes used for forming a conditional sentence: e.g. "go eastwards a mile, and you'll see it" means "if you go eastwards a mile, you will see it".
The jussive, similarly to the imperative, expresses orders, commands, exhortations, but particularly to a third person not present. An imperative, in contrast, generally applies to the listener. When a language is said to have a jussive, the jussive forms are different from the imperative ones, but may be the same as the forms called "subjunctive" in that language. Latin is an example where the jussive is simply about certain specific uses of the subjunctive. Arabic, however, is an example of a language with distinct subjunctive, imperative and jussive conjugations.
The potential mood is a mood of probability indicating that, in the opinion of the speaker, the action or occurrence is considered likely. It is used in Finnish, Japanese, in Sanskrit, and in the Sami languages. (In Japanese it is often called something like tentative, since potential is used for referring to a voice indicating capability to perform the action.)
In Finnish, it is mostly a literary device, as it has virtually disappeared from daily spoken language in most dialects. Its affix is -ne-, as in *men + ne + e -> mennee "(she/he/it) will probably go". In English, it is formed by means of the auxiliaries may, can, ought, and must: "She may go.".
A few languages use a hypothetical mood, which is used in sentences such as "you could have cut yourself", representing something that might have happened but did not.
The inferential mood is used to report unwitnessed events without confirming them. Often, there is no doubt as to the veracity of the statement (for example, if it were on the news), but simply the fact that the speaker was not personally present at the event forces them to use this mood.
In the Balkan languages, the same forms used for the inferential mood also function as admiratives. When referring to Balkan languages, it is often called renarrative mood; when referring to Estonian, it is called oblique mood.
The inferential is usually impossible to be distinguishably translated into English. For instance, indicative Bulgarian (toy otide) and Turkish o gitti will be translated the same as inferential (toy otishal) and o gitmi? -- with the English indicative he went. Using the first pair, however, implies very strongly that the speaker either witnessed the event or is very sure that it took place. The second pair implies either that the speaker did not in fact witness it take place, that it occurred in the remote past or that there is considerable doubt as to whether it actually happened. If it were necessary to make the distinction, then the English constructions "he must have gone" or "he is said to have gone" would partly translate the inferential.
The interrogative (or interrogatory) mood is used for asking questions. Most languages do not have a special mood for asking questions, but exceptions include Welsh, Nenets and Eskimo languages such as Greenlandic.
Linguists also differentiate moods into two parental irrealis categories: deontic mood and epistemic mood. Deontic mood describes whether one could or should be able to do something. An example of deontic mood is: She should/may start. On the other hand, epistemic mood describes the chance or possibility of something happening. This would then change our example to: She may have started. To further explain modality, linguists introduce weak mood. A weak deontic mood describes how a course of action is not recommended or is frowned upon. A weak epistemic mood includes the terms perhaps and possibly.
Pingelapese is a Micronesian language spoken on the Pingelap atoll and on two of the eastern Caroline Islands, called the high island of Pohnpei. e and ae are auxiliary verbs found in Pingelapese. Though seemingly interchangeable, e and ae are separate phonemes and have different uses. A Pingelapese speaker would choose to use e when they have a high degree of certainty in what they are saying and ae when they are less certain. This therefore illustrates that e and ae are mood indicators. They have no effect on the direct translation of a sentence, but they are used to alter the mood of the sentence spoken. The following example shows the difference between e and ae when applied in the same sentence.
Ngaei rong pwa Soahn e laid.
'I heard that John was fishing (I am certain about it).'
Ngaei rong pwa Soahn ae laid.
'I heard that John was fishing (but I am not certain about it).'
The use of ae instead of e can also indicate an interrogative sentence. This is a form of non-declarative speech that demonstrates the speaker has no commitment to the statement they are saying. The following sentence is an example.
Soahn ae laid?
'Does John fish?'
The language we know as Reo Rapa was not created by the combination of 2 languages, but through the introduction of Tahitian to the Rapa monolingual community. Old Rapa words are still used for the grammar and structure of the sentence of phrase but most common context words were replaced with Tahitian. The Reo Rapa language uses Tense-Aspect-Mood (TAM) in their sentence structure such as the imperfective TAM marker /e/ and the imperative TAM marker /a/.
Mortlockese is an Austronesian language made up of eleven dialects over the eleven atolls that make up the Mortlock Islands in Micronesia. Various TAM markers are used in the language. Mood markers include the past tense hortative (marking encouragement or to urge) aa, the hortative k? which denotes a polite tone, min or tin to stress the importance of something, and the word t? to denote warning or caution. Each of these markers is used in conjunction with the subject proclitics except for the aa marker. 
From SIL International: