條件句分為兩部分：表示 條件 (condition) 的 if 子句和表示 結果 (result) 的主要子句。
- 陳述一般事實、科學真理
敘述這一些已知的事實，不論過去、現在、未來，是不變的，因此都以現在簡單式表示。
If the sun shines all day, it gets hot.
If you heat water to 100 degrees, it boils.
If there is no air, wood doesn't burn.
If iron gets wet, it rusts.
Unless he asks you politely, refuse to do any more work for him.
When I finish an article, I always ask Jane to read it through.
- 說話者認為總是或未來必發生、極有可能實現之結果。
If you study hard, you will pass the exam.
If you leave now, you will catch the train.
If it rains tomorrow, the picnic will be canceled.
If you don't hurry, you might miss the flight.
If you start the homework now, you can finish it by 8.
If we ate out at all, it was always in a Chinese restaurant.
- 說話者認為未能實現之結果或表示與目前情況相反之結果。
If I started my own business right after graduation, I might be a millionaire now.
If Tom works this hard in all courses, he would not fail this semester.
If I were you, I would buy the house immediately.
If I were you, then I would not do that.
If I didn't spend money on lottery, I could buy a house now.
- 句中敘述過去之情況並沒有發生。
If I had studied hard, I would have passed the exams.
If we had started earlier, we would have met the deadline.
If I had know you were coming, I would have prepared some dessert for you.