The main verb is also called the lexical verb. This term refers to the important verb in the sentence, the one which shows the action of the subject. Main verbs can stand alone, or they can be helped along by some other little verbs (a.k.a. auxiliary verbs).
I was happy.
In this sentence, was is the main verb.
I was feeling happy.
In this sentence, feeling is the main verb and was is the auxiliary verb.
I must have been feeling happy.
Feeling is still the main verb.
If I were to be fired, I would be unhappy.
Where did they go?
What are we going to do?
Be, have and do can be used as main verbs, even though they don’t involve physical activity.