1.(表示条件)如果 ,主句用将来时,if从句用现在时表示将来 He will come if you invite him. 如果你请他,他会来的.

2.(表示虚拟)假如,要是

a.同现在事实相反的假设. 句型 ：条件从句 主句 一般过去时 should( would) +动词原形 If they were here,they would help you.

b.表示于过去事实相反的假设.

The rice would not have been burnt if you had been more careful.

If my lawyer had been here last Saturday,he would have prevented me from going.

If he had come yesterday,I should / would have told him about it.