since+时间点,比如since 1980,since 6 this morning
since+从句,表示自从...比如since he came back;since we met with each other first time
for+时间段,比如for 6 hours;for two weeks
for不可以接从句.
for+时间段,用于现在完成时,其谓语动词不可以用间隙动词,since也是如此