When do I use the imperfect and perfect tense?

The perfect and imperfect are both past tenses, but they are used to say different things:

 The perfect tense is used to convey completed actions in the past. 

eg. I walked to school yesterday.

The imperfect tense is used for a number of things:

1. What was happening. eg. I was walking to school when I saw my friend.

2. What used to happen. eg. I used to walk to school.

3. A repeated action in the past. eg. I walked to school everyday.

4. A continuous action in the past. eg. I walked to school during the summer months.

5. A description in the past. eg. It was cold.