"since"的5种用法:1.since+过去某时间点[指具体的年,月,日,时,星期几等] eg:he has beeb here since 1999 2.since+一段时间+ago eg:ha has been here since 5years ago 3.since+一般过去时的从句 eg:many things have changed since you left 4.it is/has been +一段时间+since+从句 eg:it is 2 years since i came here 5.it is +some time+since+从句 表示"自从...以来,已经...时间了"
since,有哪几种用法?请写出具体的用法,并举例!