Use of definite article before grammatical terms

a verb used in forming the tenses, moods, and voices of other verbs. The primary auxiliary verbs in English are be, do, and have ; the modal auxiliaries are can, could, may, might, must, shall, should, will, and would.

I saw the above definition in Oxford dictionary.
Please explain to me the use of definite article "the" before "tenses", "primary auxiliary" and "modal".
Thanks in advance