法則If 子句要用現在式代替未來式例句
If it rains tomorrow, I will stay home.
如果明天下雨，我就會待在家裡。
If you can’t finish the work today, you must do it tomorrow.
如果你今天不能做完這項工作，明天你一定得完成。

法則If 子句的動詞往過去降一級，從現在變成「過去式」。
主要子句用 would/should/could/might（助動詞的過去式）+ 動詞例句
If we had wings, we could fly in the sky.
如果我們有翅膀，我們就可以在天空中翱翔。
If Jenny weren’t sick, she should be here to help you.
如果珍妮沒有生病，她就會在這幫忙你。

注意！在假設語氣中，所有 be 動詞的過去式都是用 were！☒If I was you, I would take the teacher’s advice.
☑If I were you, I would take the teacher’s advice.
如果我是你，我會接受老師的建議。

法則If 子句的動詞往過去再降一級，就是「過去完成式」。
主要子句用 would/might/should/could + have + 過去分詞例句
If Jane had listened to her father’s advice, she wouldn’t have made the mistake.
如果珍當初能夠聽從她父親的建議，她就不會犯錯了。
If I had got up earlier this morning, I could have arrived in time.
如果我今天早上再早點起床，我就能夠及時趕到。