if引导真实条件状语从句和 非 真是条件状语从句 时的区别
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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.