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for和since在现在完成时中的运用：
for表“经历”：for + 一段时间
since表“自从……以来”
① since + 一段时间 + ago,
②since + 一个时间点； ③since + 从句
He has stayed here for 3 hours.
He has stayed here since 3 hours ago.
He has stayed here since 3 o’clock.
He has taught English since he came here.
除了for, since以外，常和现在完成时连用的副词还有already, yet, ever, never, just, in the past few years, over the last thirty years, so far, up to now, since 1980等。
Most college students have learned English for more than six years.
Great changes have taken place in China in the past few years.
I have never been abroad up to now.
