Question

现在完成时since和for的用法，要具体的讲解

Answer 1

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

Answer 2

since是自从的意思， 如 I have lived in Shanghai since I came back from USA. 自从从美国回国后我一直住在上海，since 引领的是一个动作。
for 引领的是一个时间段。如 I have lived in Shanghai for ten years.
希望对你有所帮助。

Answer 3

since 后时间点。since nine o'clock. 或跟从句
for 后时间段 for nine hours.
I have been in China since two years ago= I have been in China for two years.
I have taught math since I came here