since +一段时间+ ago；since +从句；It is +一段时间+ since从句；since +过去一个时间点(如具体的年、月、日期、钟点等)。since+时间点，要求谓语动词用现在完成时，而且必须是延续性动词。since+一段时间+ago，要求谓语动词用现在完成时，而且必须是延续性动词。
since+从句，要求主句谓语动词用现在完成时，而且必须是延续性动词；从句谓语动词用一般过去时，而且是终止性动词。
It is/ has been +一段时间+since从句，意为“自从……以来已经（多久）了”。
I have stayed in that country since 1995.
从1995年以来我就在那个国家了。
They have lived in that house since two years ago.
两年前他们就住在那座房子里了。
Lily has stayed at home since she left school.
莉莉离开学校之后就一直呆在家里。
It has been three months since he moved to this city.
他搬到这个城市已经三个月了。
