A passage from Huddleston and Pullum’s A Student’s Introduction to English Grammar:*
“There is a very important distinction between a small class of auxiliary verbs and the rest, called lexical verbs. The auxiliary verbs have a number of special properties. One is that they can sometimes precede the subject. This occurs in interrogatives:
AUXILIARY VERB LEXICAL VERB
a. Can you speak French ? b. * Speak you French?
|AUXILIARY VERB||LEXICAL VERB|
|a.||Can you speak French ?||b.||* Speak you French?|
Although [b] is ungrammatical, there is a way of forming an interrogative corresponding to the clause You speak French: the auxiliary verb do is added, so the interrogative clause has an extra word: Do you speak French ?
Auxiliaries are usually followed (perhaps not immediately) by another verb, as can and do in the foregoing examples are followed by speak. Notice also It will rain; They are working in Paris; She has gone home. The words will, are, and has are all auxiliary verbs.”
[The color blue, used for emphasis, does not appear in the original.]