if

如果、假如、要是

If it rains tomorrow, we will cancel the picnic.

如果明天下雨，我们就取消野餐。

是否、有没有

She asked if I had finished the report already.

她问我报告是不是已经写完了。

虽然、尽管

It was a useful if rather expensive lesson for him.

这对他来说是个有用、虽然代价不小的教训。

每当、只要

If I drink coffee after dinner, I can't sleep.

我只要晚餐后喝咖啡就睡不著。

即使、就算

I'll get this finished tonight if it takes all night.

就算要熬一整夜，我今晚也要把这件事做完。

不确定因素、疑问、条件常用复数

There are still too many ifs in this business plan.

这份营运计划里还有太多不确定因素。