非常简单.
1 延续性动词.比如说：
for + 时间段------ for five years
since+时间点------ since Sunday
till +时间点 ------- till next week / till last week
2 非延续性/ 瞬间动词,比如说：
not -- - - until- - - - ---------not stop until she arrives
时间段+ago --------- The dog died two days ago
特指的过去时间--------He came last week
when + 瞬间动词,延续性动词----- when she came,i was waiting.
while +延续性动词,瞬间动词------While i was waiting,she came.