赖世雄教你学英语语法
副词
3-5:ago, before, since, after
1．aga表“距现在若干时间以前”，时态用过去时
例：he came here a few days ago.
（他几天之前来过这儿。）
how long ago did he come?
（他多久之前来的？）
比较：he had finished the work 2 hours ago.(x)
---- he finished the work 2 hours ago.（o）
2．before表“距过去某时若干时间以前”，时态用过去完成时。
例：he told me that he had seen mary a few days before.
（他告诉我几天之前曾见过玛丽。）
比较：he told me that he had seen mary a few days ago.(x)
理由：ago只能用在过去时的句子。
注意
ago不能单独使用，而before却可以。即before可单独做副词，修饰句中的动词，通常与现在完成时、过去完成时以及过去时并用。
例：i met him ago.(x)
i met him 2 hours ago.(o)
i met him before.(o)
i have met him before.(o)
i told him that i had met him before.(o)
3．since 自从……
(1)做连接副词，引导过去时的副词从句，修饰时态为现在完成（进行）时或过去完成（进行）时的主句。
例：i have studied english since i moved here in 1971.
（自从1971年搬来这里后，我便一直学英语。）
he told me that he had studied english since he moved here in 1971.
（他告诉我自从1971年搬来这里后，他便一直学英语。）
(2)做副词用，修饰现在完成（进行）时,过去完成（进行）时的主句。since译成“之后”，可置于句尾，或句中have, has, had完成时助动词之后。
例：i moved here in 1971 and have studied english since.
= i moved here in 1971 and have since studied english.
（我于1971年迁到此处，之后就一直学英语。）
注意
since做副词时，句型结构一定为：
主语+过去时动词+明确时间副词短语+and+主语+have(has, had)+(since+过去分词）/(过去分词+since）
例：he left town in 1980 and i haven't seen him since.
（1980年他离城后，我便没再看到他。）
4．做介词用，之后加明确的时间名词做宾语，亦可用动名词做宾语
例：i have been studying english since moving here in 1971.
（1971年搬到这里后，我便一直学英语。）
i have been studying english since 1971/friday.
（自从1971年／星期五以后，我便一直学英语。）
比较：i have been studying english since 10 years.(x,ten years不是明确时间名词。）
改正：i have been studying english since 10 years ago.(o)
注意
since之前亦可用副词ever修饰。
例：ever since he came here, he has been getting along well with mary.
（自从他来这里后，便和玛丽相处得很好。）
he has stopped learning english ever since five years ago.
（自5年前，他便停止学习英语。）
he failed the exam two days ago and has felt discouraged ever since.
（两天之前他考试没及格，从此便泄了气。）
5. after 之后
after做副词时亦译成“之后”，但与since做副词时不同的是：since修饰完成时，而after则修饰过去时，此时等于later或afterwards。
例：he fell ill on monday and died three days after/later/afterwards.
（他星期一生病，3天后就死了。）
soon after, he moved to the united states.
（不久之后，他便搬到美国去了。）
the war ended in 1945, and they lived happily ever after.
（战争于1945年结束，从此他们便快乐地生活着。）