零条件句：用于陈述实际的情况，一般讨论真理，比如科学现象。用if/when提示，主从句都用一般现在时。

第二条件句：谈论不可能或者假设性的情况，以及该情况可能会有的结果。第二条件句的时间指的是现在或未来，情况是假设的。句式：if + past simple, would + infinitive。

If you invited her, she would go with you.

一些情况下，would可以有could might替代。could表示能够做某事或是一种委婉礼貌的建议；might则表示一种可能性或不确定性。

If you didn't want to go, you could give her your ticket.

If I didn't have a car, I might take the bus to work.

当if之后跟的是I/he/she/it时，通常使用were而不是was，尤其是短语“If I were you...”中