since 意为“从……以来”，主要有以下四种用法。

1. 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。

2. 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。

3. 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。

4. It is/ has been +一段时间+since从句，意为“自从……以来已经（多久）了”。

Example: I have stayed in that country since 1995. 从1995年以来我就在那个国家了。

Example: They have lived in that house since two years ago. 两年前他们就住在那座房子里了。

Example: Lily has stayed at home since she left school. 莉莉离开学校之后就一直待在家里。

Example: It has been three months since he moved to this city. 他搬到这个城市已经三个月了。