since 過去式(起始時間,動作) + 現在完成式

since    自……以來，從……至今

since 過去式(起始時間,動作)  +  現在完成式

It ' s been ten years since they married .
他們結婚到現在已經十年了。

She has been living a hard life since her husband died .
自從丈夫死後，她一直過著艱苦的生活。

既然；因為，由於
He must have taken the book since it isn ' t here .
他一定是把書拿走了，因為書已不在這裡了。

Since it is so hot , let ' s go swimming .
既然天氣這麼熱，我們去游泳吧。

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for +  一段時間 , 現在完成式

Mary has watched TV for three hours.

Mary 已經看了三個鐘頭的電視了。

Have you ever been to London ?   你到過倫敦嗎？

He is one of the very best men I have ever met .   

他是我遇見的最好的人之一。

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We toured France about three years ago .  
我們大約三年前去法國遊玩過。

三年前去過法國，但不知道玩多久。(曾經)

若用 for since 就可以明顯知道玩多久

We have toured France for three years.(已經)