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。)