1. 比较since和for Since 用来说明动作起始时间，for用来说明动作延续时间长度。 I have lived here for more than twenty years. I have lived here since I was born.. My brother has been in the Youth League for two years. I have not heard from my uncle for a long time.
注意：并非有for 作为时间状语的句子都用现在完成时。 I worked here for more than twenty years. 我现在已不在这里工作。 I have worked here for many years. 现在我仍在这里工作。
2. since的四种用法 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+ ago I 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.
