Question

It is +一段时间＋since /before/until的区别？

还有道题：
the soldier devoted most of his time to the security of the country. and it has been three years____he went home.
A.before B.after C.since D.until
为什么要选C?

Answer 1

用since.时，前面一般是现在完成时，表示自从...时候已经完成的动作； before 时，其前面一般根据before后面的时态来判断，若before后面的时态是一般现在时（before引导时间状语从句，这里一般现在时表示一般将来时），其前面就是一般现在时，若是过去式，那么其前面就是过去完成时,即发生在过去的过去； until表示直到的意思，其前面一般为一般将来时，通常与not连用，构成not...until,直到...才...的意思。根据这句话it has been three years____he went home.前面是现在完成时，所以填since.
我给你几个句子，It has been two years since I graduated from my high school.
自从我高中毕业，已经两年了。 It is still a year before I graduate from my college.
在我大学毕业之前，仍然还有一年的时间。 I had finished my homework before it was 9 o`clock last night. 在昨天晚上9点之前，我就已经完成了我的作业。 I did not finish drawing the picture until it was 9 o`clock last night. 直到昨天晚上9点我才画完这幅画。

Answer 2

靠语感，三言两语解释不了