第1个回答 2009-05-30
since是在...之后,从..以后/以来
before 是在...之前
你说的那两个词都是在句中做连词
It is a long time before the building was accomplished.
这个建筑花了很长时间才建立起来
It has been a long time since Susan said that she want to give up smoking.
离Susan表示她想戒烟已经过了很久了
第2个回答 2009-05-30
如果用before,谓语应该用过去时。而since一般用现在完成时。前者强调过去怎样,后者强调过去的一段时间多现在造成的影响。
before:在,,,之前
since:自从,,,
第3个回答 2009-05-30
it is 5 days since i left the hospital.之后
it is 5 days before i left the hospital.之前