你好，1) since +过去一个时间点(如具体的年、月、日期、钟点、1980,last month,half past six).I have been here since 1989.2) since +一段时间+ agoI have been here since five months ago.3) since +从句Great changes have taken place since you left.Considerable time has elapsed since we have been here.4) It is +一段时间+ since从句It is two years since I became a postgraduate student.since的用法主要有两点：一是表示“自从……以来”,二是表示“既然”之意.但如果只了解词的含义,而不了解其用法,在使用时就会造成错误.例如：I have not heard from him since I lived in Shanghai.for +时间段，for“累计、达”,后面跟时间段,不可以跟时间点.所以是，for four hourssince four hours ago希望对你有帮助~
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