Auxiliary verbs are sometimes called helping verbs because they are needed to form many of the tenses. The most used auxiliary verbs are the verbs to be, to do and to have. For example: the auxiliary to do is needed to ask questions in the present and past simple tenses. To be is needed for the present and past continuous, and all the passive forms. The auxiliary to have is used in the present and past perfect tenses. Some tenses, like the present perfect continuous, need more than one auxiliary!
Here are some examples of the common auxiliary verbs in action:
Other common auxiliary verbs are: will, should, would, can, must, might, may, could (These verbs are often called modal verbs).
The auxiliary verbs are very often contracted. For example, you can say I'm playing tennis tomorrow, instead of I am playing tennis tomorrow. The contracted form (which needs an apostrophe) is more likely to be used in spoken language*. So, for example, it would be a little unusual to ask your friend: Why did you not call me yesterday? More probable is: Why didn't you call me ..?
On the other hand, if you are doing a written report or essay, it is more usual to write the full form: Why did America not join the war until 1941? (instead of: Why didn't America ..?).
The short form itself often has two alternatives, which are equally usual and correct. For example:
* The full form is common in spoken language when the speaker wants to emphasize what she is saying. For example: