later、after、ago、before的用法：
①“一段时间 later/ago”分别表示“(多久)以后/以前”,主要用于过去时态。
②“after/before 某个时刻”分别表示“在某时刻之后/之前”,此时两个词是介词。
③ago与before：ago只能用于过去时,before用于完成时。
如：He had an accident a week ago.(一周前出了一个事故)/
Some years later, the boy became a very famous singer.(数年后这个男孩成了著名的歌唱家)/ Have you been there before?(你从前到过那儿吗？)/
After a few years he gave up smoking.(过了几年他戒了烟。)
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