现在完成时中可以用for, since，而且很常用：
I have studied English since 3 years ago.
I have studied English for 3 years.
2. for, since可以与延续性动词连用：
I have kept the book for 4 days.
I have kept the book since 4 days ago.
3. 延续性动词能再现在完成时用，就比如
I have kept the book since 4 days ago.
for 表原因 since 时间连词，组合起来是“为了自从”
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