1、later“…时间后”结构：时间段+later

　　常用于一般过去时。They went to Beijing five days later.

　　(later单独在句尾，常用于将来时：I’ll see you later.)

　　2、after“…时间后”结构：after+时间段，常用于一般过去时，和1相同。They went to Beijing after five days.

　　(after也可加句子：I’ll send you an e-mail after I get home. He found out the information after he had searched the Internet for thirty minutes.)

　　3、ago“…时间前”结构：时间段+ago, 用于一般过去时。The Greens moved to Shanghai four weeks ago.

　　(since +时间段+ago，主句用现在完成时)

　　4、before 单独放在句尾，常用现在完成时：“以前”

　　I have been to London before. He has seen the film before.

　　(若是时间段+before, 则常用过去完成时，译为“…时间前”：

　　I had seen the film two weeks before. We had found out the answer to the problem an hour before.)