Артикль а и an когда правильно применять

1. i made a terrible ( serious )mistake when i had made this telephone call.

2.would you like to make a cup of tea for me?

3.i always make important decisions.

  4.i don't want to do my homework.

  5. he made a great discovery.

john is helped by mary.

the office is cleaned every day.

this bookstore was built two years ago.

this parcel has been brought by the courier.

the car is being washed.