What is the helping verb in sentence have you ever written a letter before?

The helping verb in the sentence "Have you ever written a letter before?" is have.

Here's why:

* Helping verbs assist the main verb in a sentence to convey information about tense, mood, and voice.

* In this sentence, the main verb is "written", and it's in the past participle form.

* "Have" is used with the past participle to form the present perfect tense, indicating an action that started in the past and continues to have relevance in the present.