the children (not to learn ) the poem yet

The children have not learnt the poem yet.

The sentence is in Present Perfect because there is yet in the sentence. Present Perfect is formed with the help of auxiliary verb to have and the third form of irregular verb and if verb is regular we add – ed.

I have learnt. (irregular verb).

She has washed. (regular verb).

Has is used for the third person singular.

Negative is formed —  have / has  + not + the third form of irregular verb.

He has not driven his car.

Interrogative is formed — have / has + subject + predicate.

Have we been to Paris?