"since"后面通常接 现在完成时。例如：
1. I have been here since 1990.
2. It is one year since I became a postgraduate student.
3. I have been to primary school since 2000.
"since"后面也可以接"一段时间+ago"，但这种情况实际上也是现在完成时的另一种表达方式，因为"ago"表示从过去某个时间点到现在的时间段，而"since"则强调从那个时间点开始一直持续到现在。例如：
1. I have been here six months ago.
2. I have been here six years ago.
需要注意的是，"since"后面接从句时，从句通常用一般过去时，例如：
1. Great changes have taken place since you left.
2. It is one year since I became a postgraduate student.
总结：
"since"后面通常接现在完成时。
"since"后面也可以接"一段时间+ago"，这实际上也是现在完成时的另一种表达方式。
"since"后面接从句时，从句通常用一般过去时。