"If " 有四種不同的用法

Conditional Sentences

Type 1 --- Facts

e.g. If you heat ice, it melts.

Type 2 --- Real/Possible Situations

e.g. If you work, you will pass the exam.

Type 3 --- Imaginary

e.g. If I were you, I would invite Mary to the party.

Type 4 --- Describing imaginary situations in the past/Could happen in the past but did not

e.g. If I had worked harder, I would have passed the exam.

e.g. If I had done my homework, I wouldn't have punished by Miss Chan.