1、从副词ago 和before 说起
ago 指从现在算起的以前,如：5 hours ago 指从现在算起的5小时以前,ages ago 指从现在算起的几个世纪以前（即很久很久以前）
before指从过去某个时间算起的以前,即在那之前或早些时候,如：the day before指从那一天（the day）算起的以前（即前一天）.这一点从the day before yesterday（前天）就可以看出,它不是指今天的前一天昨天,而是从昨天算起的前一天.同理,5 hours before意思是过去某个时间的5小时前,到现在已经不止是5小时,可能是6小时以前,10小时年以前,或者数日、数十日以前.
2、由于since 5 hours before 和since 5 hours ago 所指的概念不同,它们所用的时态也就不同.
since 5 hours ago 与现在完成时连用,如：They haven’t seen each other since 5 hours ago【他们从（现在算起的）5小时以前就没再见面.】
since 5 hours before 与过在完成时连用,如：He said thet hadn’t seen each other since 5 hours before.【他说他们从（said那个时间算起的）5小时以前就没再见面】
3、凡是含有表示时间的since的句子,除了动词be 以外,一律都得用完成时态（现在完成时或过去完成时）,如：
It (is) three years since we were there.我们在那儿已经呆了三年.
We (have both changed) since we parted.分别以来我们彼此都变了.
The two soldiers who passed away years ago had been friends since childhood.那两位数年前故去的军人从幼时起就一直都是好朋友.
4、下面的句子语法不都正确
1）The air conditioner was turned on 5 hours ago 没问题,5 hours ago 表示过去时间,因此谓语动词应该用过去时态.
2）The air conditioner has been turned on since 5 hours before不正确,5 hours before表示过去某个时间的5小时以前,而谓语动词has been turned on却只从过去某个时间到现在的状态,一个指的过去的过去,一个指从过去到现在,时间没有交汇点,二者存在矛盾.因此应该改为——.
* The air conditioner has been turned on since 5 hours (ago).
* The air conditioner (had) been turned on since 5 hours before谢谢前辈的回答，细致精确。 有几个句子，拿不准怎么翻译好，网上没查到合适的，劳烦您帮忙翻译下吧。1A 是B的 几分之一？ 2A 是B的几倍？3A 是B的五分之一4A 是B的5倍。5A的工资是B的两倍6这条河流的长度是那条的3倍 O(∩_∩)O谢谢您啦。1A 是B的 几分之一？ What a fraction of B is A?2A 是B的几倍？What times of B is A?3A 是B的五分之一 A is one fifth of B. 4A 是B的5倍。A is as five times big as B. / A is four times bigger than B.5A的工资是B的两倍A's pay is as twice more as B'6这条河流的长度是那条的3倍 The length of this river is as three times long as of that