难点提示:用条件状语从句时要注意时态的正确使用,当主句是将来时的时候,从句要用一般现在时.He will not leave if it isn’t fine tomorrow.一般将来时, 一般现在时They are going to have a picnic if it doesn’t rain next week.一般将来时, 一般现在时而主句为情态动词作为谓语时,从句要用一般现在时。主句为祈使句,从句要用一般现在时。即所谓主将从现,主情从现,主祈从现。但在虚拟语气中就不同了:1.与现在事实相反 主句过去将来时,从句过去时 可记为 if did , would do2.与过去事实相反 主句过去将来完成时,从句过去完成时 if had done , would have done3.与将来事实相反 主句过去将来时,从句过去将来时 if should do ,would do 或 if were to do ,would do 或 if did , would do 详见虚拟语气词条。要点: 由连词because, since, as引导, 也可由 now that 等词引导