since和because均为从属连词，引导时间状语从句。表示已经知道的原因时用since，即某种原因在说话人看来已经很明显，或已为听话人所熟悉，通常被置于语句之前。because引导的从句表示直接而明确的原因和理由，表示因果关系。
since的用法
1、接时间点：如 I have stayed in that country since 1995. 要求：谓v.用现在完成时，而且须是延续性动词
2、接一段时间 + ago：如 They have lived in that house since two years ago. 要求：谓v.用现在完成时，而且须是延续性动词。
3、接时间状语从句：如 I have loved her since I met her last time. / Lily has stayed at home since she left school.
要求：主句的谓v.用现在完成时，而且须是延续性动词；从句的谓v.用一般过去时，而且须是终止性动词。
4、用于句型“It’s / It has been + 一段时间 + since + 时间状语从句”中，译作“自从…以来, 已经…(时间)了”。
如：It’s / has been three months since he moved to this city.
要求：主句的谓v.用一般现在时或现在完成时；从句的谓v.用一般过去时，而且须是终止性动词。
5、用于句型“一段时间 + has passed since + 时间状语从句”中，译作“自从…以来, …
(时间)已经过去了”。还可写成：Three months has passed since he moved to this city.
要求：主句的谓v.用现在完成时，而且动词必须是has passed；从句的谓v.用一般过去时，而且须是终止性动词。
