The imperfect tense is used to describe past actions that were repeated, habitual, or took place over an extent of time.

• I lived in Britain for many years.
• She used to study French.
• We kept asking questions.

The perfect tense is used to describe past actions that are sudden or momentary or if the speaker wants to emphasize an action as completed.

• Suddenly we caught sight of the island.
• I have read that book.
• She learned English.