auxiliary verb
Definition - (Aka helping verb | helper verb | verbal auxiliary | linking verb)
A verb that modifies the voice, tense, or mood of another verb.

Example -
I have written a letter (The auxiliary verb have modifies the tense of the verb written.)

Etymology -
The phrase derives from the Latin auxiliaris, helpful + verbum, verb or word.

Example Usages -

1. "Our auxiliary verbs give us a power which the ancients, with all their varieties of mood, and inflections of tense, never could attain."
(Source: OED: 1834 Southey, Doctor 1 )

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