since的用法：译作“自从…以来”
(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...
