How do we decide whether to use the perfect or imperfect in the past tense in French?

This is one of the trickier aspects of French grammar. The first thing we must do is to decide whether we definitely need to use the past tense in French. The trickiest aspect is then knowing whether to use the perfect or imperfect tense. The imperfect tense is used in several situation, the first being when describing something in the past (the tree was green); the second main use of the imperfect is when describing events in the past that were habitual (I used to play football - the "used to" in English is a good clue here); the final main use is when talking about INCOMPLETE past actions (I was eating... - this often corresponds to the "I was -ing" in English).The perfect tense is concerned mainly with one-off, singular events in the past. It could be described as being for COMPLETE past actions (last week I played football). The best way of contrasting the perfect and imperfect is seeing them both in one sentence. "I was reading the newspaper when he arrived." The first part of this sentence must be in the imperfect (I was reading) because this is an INCOMPLETE event in the past. The second part (when he arrived) must be in the perfect as this is a singular, one-off, COMPLETED event.