Question

Which word in the sentence is the auxiliary verb?

I did mail your letter for you.

A.
did

B.
letter

C.
mail

D.
you

Answer 1

A. did
"It is also common to use do, does and did asauxiliary verbs (or helping verbs) together with another verb in its base form. This is used to create negative sentences, questions, or for adding emphasis."

Answer 2

a) did

Step-by-step explanation:

Given sentence,

→ I did mail your letter for you.

Then the auxiliary verb is,

→ I did mail your letter for you.

→ [ did ]

Hence, the option (a) is correct.