A/AN and THE

1. a

1.1. one of many of that kind and not a specific one

1.2. used when noun starts with a consonant

1.2.1. Tim sat down on a chair.

1.3. used with adverbs of definite frequency

1.3.1. once a week, three times a day

1.4. used with words showing quantity

1.4.1. 1 pound a kilo

2. an

2.1. one of many of that kind and not a specific one

2.2. used when noun starts with a vowel

2.2.1. Tim ate an apple.

3. the

3.1. when thinking of a specific thing

3.1.1. Tim sat on the chair nearest to the door.