如果 if（如果） 从句用一般现在时，主句应该用一般现在时表示现在或者经常性的情况，不能用一般将来时；用一般将来时表示将来的情况。例如：

    Oil floats if you pour it on water. 如果你把油倒进水里，油会浮在水面上。

    I'll give her your love if I see her. 要是我看到他，我会代你向她问好。

    （摘自《牛津英语用法指南》第四版238）

在含if条件句的复合句中，如果说话者侧重说明主句的谓语表示经常性或习惯性的行为，常用一般现在时。例如：

If I do not understand what he says, I always ask him. 我不懂他的话时, 总是去问他。《现代英汉综合大辞典》

If I go to bed late I feel dreadful in the morning. 我如果睡得晚﹐第二天早上就感到很不舒服。《朗文当代英语词典》

I often take paracetamol at work if I get a bad headache. 工作中头疼得厉害时，我常服用扑热息痛。《柯林斯高阶英汉双解词典》

RE: 这里从句用一般现在时，主句也可以用一般现在吗?

这是英语里一个基本的语法现象，注意一下人家的术语。

Zero Conditional

for certainty

If you heat ice, it melts.

We use the so-called zero conditional when the result of the condition is always true, like a scientific fact.

Take some ice. Put it in a saucepan. Heat the saucepan. What happens? The ice melts (it becomes water). You would be surprised if it did not.

Notice that we are thinking about a result that is always true for this condition. The result of the condition is an absolute certainty. We are not thinking about the future or the past, or even the present. We are thinking about a simple fact. We use the Present Simple tense to talk about the condition. We also use the Present Simple tense to talk about the result. The important thing about the zero conditional is that the condition always has the same result.

Look at these example sentences:

We often use when instead of if, for example: When I get up late, I miss my bus.

-----------------------

再如：

The Zero Conditional

We can make a zero conditional sentence with two present simple verbs (one in the 'if clause' and one in the 'main clause'):

• If + present simple, .... present simple.

This conditional is used when the result will always happen. So, if water reaches 100 degrees, it always boils. It's a fact. I'm talking in general, not about one particular situation. The result of the 'if clause' is always the main clause.

The 'if' in this conditional can usually be replaced by 'when' without changing the meaning.

For example: If water reaches 100 degrees, it boils. (It is always true, there can't be a different result sometimes). If I eat peanuts, I am sick. (This is true only for me, maybe, not for everyone, but it's still true that I'm sick every time I eat peanuts)

Here are some more examples: