if 引导的条件状语从句和时间状语从句中常用一般现在时代替将来时，如:
If it rains tomorrow, I won't go there.
如果明天下雨，我就不去那儿。
I will call you as soon as I arrive there.
我一到那儿就给你打电话。
I won't go to the party unless i am invited.
除非我被邀请，我将不去参加那个聚会。
I will buy you a MP4 as a present as long as you behave well.
只要你表现好，我就会给你买MP4做礼物。
条件句把握一个原则：主句用一般将来时态，if从句用一般现在时态，简称“主将从现”，例如：
It may rain tomorrow.
I will go for a picnic.
----I will go for a picnic if it doesn't rain tomorrow.
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