If you had worked harder, you would have passed your exams.
要是你再努力一点，就通过考试了。
What would happen to your family if you were to die in an accident?
要是你在意外事故中身亡，你的家人会怎么样呢？
If Dad were here, he would know what to do.
如果爸爸在这儿，他会知道怎么做的。
Taste the soup and add salt and pepper if necessary.
尝一下汤，需要的话加一点盐和胡椒粉。
I want to get back by five o’clock if possible.
如果可能的话，我想5点之前回来。
I think I can fix it tomorrow. If not, you’ll have to wait till Friday.
我想明天我能修好它，如果不行，你只好等到周五了。
The missiles can be fired only if the operator types in a six-digit code.
只有在操作员输入一组六位数密码后，导弹才能发射。
We’ll face that problem if and when it comes along (= if it happens or when it happens ).
如果这一问题真的发生了，我们就要面对它。
If by any chance you can’t manage dinner tonight, perhaps we can at least have a drink together.
万一你今晚不能来吃饭，或许我们至少可以一起喝一杯。
n. 条件；设想
condition、
qualification、
conception、
assumption