since可以放在句首，而for不可以。另外，since是点时间，相当于when的意义；for是段时间，相当于while的意义！ It has been a long time since we last saw each other. We haven't saw each other for quite a long time.
We've got some food to eat for tomorrow already. 我们已经有明天吃的食物了。 Since we've got plenty food in the fridge, you don't have to worry about us for tomorrow. 你不用替我们担心明天，因为我们有很多食物在冰箱里。
第3个回答 2019-01-01
since + [过去的]年分 / 月分 / 季节 since 2014 ; since last month; since february / since this spring / since last summer / since the 80's for + 时间/ 年 / 月 / 日 for three hours ; for five years ; for two months ; for four days ; for many years
第4个回答 2008-05-03
for加时间段 since加时间点 I've been a student for 10 years/since 10 years ago