Question 3 Which do you think is the main verb in this sentence?
The mother scolded the child who had been annoying her.
Scolded is the main verb. The other verb, had been annoying, is subordinated by the relative pronoun, who. You can see how this business of subordination works if you turn the sentence round:
The child annoyed her mother who had been scolding her.
Why is recognising the main verb so important?
It represents the most important part of the sentence, the most important thing that happens.
If you can speak English then you can understand English.
If you understand English then you will use it more effectively.