初二 英语 快 请详细解答,谢谢! (8 21:15:6)

在现在（过去）完成时中，两者都是表示“一段时间”。
具体地说，for + 一段时间 ，是能够直接看得出的一段时间，如：for a year,for two months； since在表示“一段时间”时，其本意是指“从过去某一时间点到‘现在’的一段时间，如果作连词使用，其从句中的谓语动词必须具备两个特点：过去时，终止性。
例：He has lived in Beijing for a week.
He has lived in Beijing since a week ago.
He has lived in Beijing since he came here.

祝你进步！

前者跟段时间 for a year，
没有时态要求。
后者跟一个时间点 如： since he came back。
通常跟完成时态连用。

Since 用来说明动作起始时间，for用来说明动作延续时间长度。
I have lived here for more than twenty years.
I have lived here since I was born..
My aunt has worked in a clinic since 1949.
Some new oilfields have been opened up since 1976.
I have known Xiao Li since she was a little girl.
My brother has been in the Youth League for two years.
I have not heard from my uncle for a long time.

注意：并非有for 作为时间状语的句子都用现在完成时。
I worked here for more than twenty years.
(我现在已不在这里工作。)
I have worked here fo