if引导真实条件状语从句,从句用一般现在时表示将来,主句用一般将来时.
if it doesn't rain tomorrow, we will go shopping.如果明天不下雨,我们就去购物.
if引导非真实条件状语从句要用虚拟语气.
1. 表示与现在事实相反或不可能发生：条件状语从句：一般过去时（虚拟语气中be→were）
主句用：would(should, could, might)+动词原形.
If we had time now, we would read it again.
If I were you, I would work hard.
2. 表示与过去事实相反或不可能发生：条件状语从句：had+过去分词；
主句：would(should, could, might)+have+过去分词.
If he had taken my advice, he would have succeeded in the test.
If I had known your telephone number then, I would have called you.
3. 表示与将来的事实可能相反或不可能发生：条件状语从句：①一般过去时②should +动词原形③were to+动词原形；主句：would(should, could, might)+动词原形.
If it should rain, the crops would be saved.
If it were to snow tomorrow, they would not go out.