what are main verbs ?

As the name suggests, main verbs tell the main action or state of being of the subject. They are the important verbs in the sentence. A main verb can stand alone in a sentence or it can be supported by a helping verb. the following examples will make this clear:

I write a letter.

(main verb 'write')

I am writing a letter.

(helping verb 'am' supports main verb 'writing')

I had written a letter.

(helping verb 'had' supports main verb 'written')

As you can see from the given examples, the main verb has described the action and the helping verb has helped the main verb and described the time of the action.