英文難題~Help!

1:present perfect tense是現在完成時式:
表示动作在過去開始.但講話或寫作時剛完成
eg:I have just finished.
I have done the job.
2)past perfect tense是過去完成時式
表示动作在過去某一時間內巳经完成
eg:He went home after he had finished his work.
I never went to bed before i had done my work
3past continues tense是過去繼續時式
动作正在發生於過去某一時間內
eg:they were having dinner when Judy came.
She was knocked down by a car while she was crossing the road.
4)-to infinatives 是不定式