When to use the imperfect tense and the perfect tense

The imperfect tense and the perfect tense are often used together.

The imperfect describes what was going on/what was happening but is interrupted by the perfect (ie something that has suddenly happened):

• je mangeais quand tu as telephoné - I was eating when you called
• il faisait beau quand tu es arrivé - it was sunny when you arrived