since后面什么时候跟完成时态，什么时候用过去时态？

since后面什么时候跟完成时态，什么时候用过去时态？

1、since +过去一个时间点 (如具体的年、月、日期、钟点、1980, last month, half past six)。
I have been here since 1989.

2、since +一段时间+ ago
I have been here since five months ago.

3、since +从句
Great changes have taken place since you left.（跟一般过去时）
Considerable time has elapsed since we have been here.（现在完成时）

4、It is +一段时间+ since从句
It is two years since I became a postgraduate student.（跟一般过去时）

since+时间点，一般用在完成时态句子中。for+时间段，用在完成时态或者过去时态句子中。eg:我学英语两年了i have studied english for 2 years.i have studied english since 2 years ago.自从两年之前我就开始学英语了。i watched tv for two hours last night.我昨晚看了两个小时的电视。