Leading Auxiliaries
The very first auxiliary in a verb string plays multiple important roles relating to tense, negation, interrogatives, short answers, and more. Adjusting the structure of a sentence often comes down to the 'leading' auxiliary verb.
Tense Designation
The leading auxiliary communicates whether the verb phrase is Present or Past. See The 1 Red ‘S’ and The 1 Blue Word videos for details. These establish the rule with more basic tenses, but the rule remains with advanced tenses and complex verb constructions.
Question Formation
Inversion is the process of placing the leading auxiliary in front of the subject, and it is used most often for certain Question Forms, starting with the most basic: Yes/No Questions.
Subject-auxiliary inversion also applies to Choice Questions, Negative Questions, and Predicate Questions.
Ellipses
Ellipses are when a clause is cut short, often because the rest of the clause has already been voiced, perhaps in the preceding clause. This is most common with Short Answers (see the Y/N Questions video above), but can also be seen in Question Tags and in some Comparative formations.
Ellipses usually consist of a noun or pronoun, the leading auxiliary, the negative not or n’t (when applicable), and perhaps some particle like ‘yes’ or ‘no’. That’s it!
Remember that if there are not already any auxiliaries to work with, we use a form of DO. DO is a ‘dummy’ auxiliary, which means it carries no meaning, but only exists for structural reasons like the ones featured above.
Also, many adverbs – most notably not and never – come immediately after the leading auxiliary. The negative not is often contracted as n’t and appended to the end of the leading auxiliary. If the leading auxiliary shifts around in the sentence order, the n’t goes along with it.