Auxiliary Verbs

Auxiliary verbs are type of words that functions as complement. It means that these words are complete the sentence grammatically. One of the examples is "is".  The word "is" is used to complete structure of a sentence which consists of Single Noun and/or adjective, adverb, noun. Let's look at the following sentences:
1. Tony smart.
2. Mira in the classroom right now.
3. David a doctor.

Those expressions is understandable, however, grammatically, those expressions miss of auxiliary words. They miss the word "is". So the proper grammar for those expressions are:
1. Tony is smart.
2. Mira is in the classroom right now.
3. David is a doctor.

How is that?

Well, besides "is", there is a bunch of auxiliary words exist in the English speaking society, such as;

1. are
2. is
3. was
4. were
5. will
6. shall
7. have
8. may
9. can
10. could
11. should
12. have
13. has
14. would
15. had
16. been
17. may
18. must

The usage of each auxiliary words can be found completely in the following link.