When you conjugate a one-word verb, you can form the present tense (he decides) and the past tense (he decided) with just one verb word. But when you go beyond the present and past tenses, you need an auxiliary verb, also called a helping verb, to complete the conjugation (he will decide).
We have 16 auxiliary verbs: to be, do, have, can, may, might, must, shall, will, ought (to), used (to), should, would, could, dare, and need (to). Five of these can serve as full verbs (to be, dare, do, have, and need), that is, you can conjugate them.
Auxiliary verbs join with main verbs to show certain tenses, voices (active and passive) (The case was decided by the court), conditions (He might win), and obligations (She must study).
When joined to a main verb, auxiliaries form multiword verbs. Some people incorrectly believe that when you modify a multiword verb form with an adverb, you should not break up the multiword verb form. In the section on Adverbs in Parts of Speech on Grammar.com, you'll find a full discussion on the placement of adverbs. Click here for the beginning of that discussion.
Thus, study these examples:
She will probably win the election. He has definitely decided to attend graduate school.