since 用来说明动作起始时间，for用来说明动作延续时间长度;

　　eg：I have lived here for more than twenty years.

　　I have lived here since I was born.

　　for后面多接一段时间，而since后面则接一个时间点，若since后面也出现时间段，则在时间段后加ago;

　　eg： I have lived here since twenty years ago.

　　错句：I have worked here since many years.

　　since句型：

　　It is +一段时间+ since从句，since从句要用一般过去时;

　　eg：It is two years since I became a postgraduate student.

　　It is three years since I joined the army.