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The habitual aspect is a form of expression connoting repetition or continuous existence of a state of affairs. In English, for the present time there is no special grammatical marker for the habitual; the simple present is used, as in I go there (every day). However, for past reference English uses the simple past form or either of two alternative markers: used to as in we used to go there (every Thursday), and would as in back then we would go there (every Thursday).
The form [would + infinitive] is employed to talk about a habit or frequent action in a former time. One usually applies [would + infinitive] for the past habitual when one is telling a story about the past.
The past habitual employment of would requires an accompanying indication of the time of occurrence (more specifically than simply before the present): e.g., Last year we would go there frequently, but not simply We would go there frequently.
This application of would to mark the past habitual is distinct from each of several other uses of would: as a tense marker for future of the past (after that experience we would not try it again for the next three years); as a conditional mood marker (I would do it if I could); and as an indicator of politeness (Would you open the door, please?).
The linguistic expression used to expresses past states or past habitual actions (usually with the implication that they are no longer so), as in, I used to eat ice cream, or a state of accustomedness, as in I am used to eating ice cream.
In the first case--the past habitual one--it is followed by the infinitive (that is, the full expression consists of the verb used plus the to-infinitive). Thus the statement I used to go to college means that the speaker formerly habitually went to college, and normally implies that this is no longer the case.
Used to is typically employed without a specific indication of the time of occurrence--e.g., We used to go there frequently.
While used to expresses habitual aspect in the past rather than modality, it is not treated like a modal verb in questions or negative statements. (This usage is informal, especially when the "d" is pronounced, but no direct formal equivalent exists.) Used is used as a past tense of an ordinary verb, and forms questions and negatives using did: Did he use(d) to come here? He didn't use(d) to come here. In the habitual verb form, used is pronounced /ju:st/, in contrast to the ordinary verb use /ju:z/ and its past form used /ju:zd/ (as in Scissors are used to cut paper).
The verbal use of used to should not be confused with the adjectival use of the same expression, meaning "familiar with, accustomed to", as in I am used to this, we must get used to the cold. When the adjectival form is followed by a verb, the gerund is used: I am used to going to college in the mornings.
It is considered informal to apply used to in questions or negative sentences, as Did he use(d) to...? or He didn't use(d) to...; but this is sometimes used in informal spoken English. It is more standard to ask questions and make negative forms using simple past. Used to implies the idea that something was an old habit that stopped in the past. It indicates that something was often repeated in the past, but it does not usually occur in the present. Used to can also be used to talk about past realities or generalizations which are no longer real. Both simple past and used to can refer to past habits, past facts and past generalizations; however, used to is preferred when emphasizing these forms of past repetition in positive forms. On the other hand, when forming questions or negative sentences, modern prescriptive grammar dictates that the simple past is better.
The expression used to refers to habits or frequent actions in a former time which we are not done in the present. It is also employed to identify states in the past which are no longer true. For example:
There is a considerable difference between [used to + (verb)] and [(to be) + used to].
I used to drink black coffee means that in the past I drank black coffee, but now I don't. Used to describes an action that did happen, but does not happen now.
In contrast, I am used to drinking black coffee means that at first drinking black coffee was unusual, but now it has gotten familiar. [to be + used to] tells of a state of affairs that was unfamiliar, but that the speaker/writer is now accustomed to (also sometimes a state of affairs that was once hard and is now simple or easy). I am accustomed to black coffee has the same meaning.