since引导时间状语从句时，从句动词通常用一般过去时，主句动词用现在完成时。
since：
prep.自从;自…以后;从…以来;(表示气愤)何曾，什么时候。
conj.因为;由于;既然;从…以后;自…以来。
adv.自…以后;从…以来;此后;后来。
She's been using since she was 13.
她从13岁起就吸毒。
His quality of life has improved dramatically since the operation.
手术后他的生活质量大大改善了。
It's a long time since they left.
他们已经离开很久了。
It's been two weeks now since she called.
她到现在已经有两个星期没来电话了。
I had been out since I was 17.
我从17岁起就公开我是同性恋。
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