Inversion of the subject and the verb is required when certain negative adverbs, such as never and nowhere, and adverbs that have a broadly negative meaning, such as hardly, little, only, rarely, scarcely and seldom, come before the subject in the sentence. Another word that triggers inversion and the insertion of an auxiliary verb is nor. The constructions with subject-verb inversion are a little more formal than those with if, but are used in everyday language nevertheless.
The verbs dare (= be willing to risk), need (= have to, must), ought to and used to are often treated by grammarians as modal auxiliaries. There are a number of things to be noted about the grammar of these verbs. Dare and need may behave either as auxiliary verbs or as lexical verbs.
The auxiliary verb do is used along with a main verb to give emphasis to positive statements. Note, however, that you cannot use do in this way to emphasize the lexical verb be, nor any auxiliary verb. When parts of a sentence are omitted because they can be understood from what has already been said, this is known as ellipsis. Ellipsis avoids unnecessary repetition of information.
A tag question (or question tag) is a short question used, usually at the end of a sentence, to ask the listener to show that he or she agrees with what has been said or confirm that what has been said is true. Tag questions are generally formed with auxiliary verbs, but they may also be formed with the lexical verbs be and have.
Not and -n’t almost always follow an auxiliary verb, though they may also follow the lexical verbs be and have. Notice that -n’t is always attached to the verb it follows. In negative questions, -n’t precedes the subject of the verb whereas not follows it.
Lexical verbs cannot form questions on their own. There must always be an auxiliary verb (an operator) in a question. When there is no other auxiliary verb in the sentence, the verb do is brought in to be the operator, followed by the base form of the lexical verb.
Some auxiliary verbs have shortened forms, used both in speech and writing. Short forms are always attached to the preceding word. Short forms of verbs are mostly used after personal pronouns, such as I, you, he, they, etc. But they can be used, especially ‘s, after other pronouns, after nouns, and after certain adverbs.
Auxiliary verbs, also known as helping verbs, are used along with main or lexical verbs to form complex tenses and to indicate possibility, necessity and permission. They are usually divided into primary auxiliaries (be, have and do) and modal auxiliaries, (such as shall, will, can, may, etc).