About conditional 0-3

Zero Conditional Formulation

If/When + Present simple + Present simple 

Concept: A fact, sth that is always true

Ex: if you heat ice, It melts.

Ex: if demand for a product rises, its price rises too.

First Conditional Formulation

If + Simple Present + Simple future(will/will not/are going to/may/could/might..)

Concept: A Present/Future situation of high probability

Ex: I see her quite often.表示我经常见到她，极大可能，我明天还会见到她，so这句话就可以变成，If I see her tomorrow, I will speak to her.

Ex: They are always late.表示他们经常迟到，极大可能，明天他们还会迟到，so这句话就可以变成，I will not let them in if they are late again.

Second Conditional Formulation

If + Simple Past + Could/Would do(not do) +Base form

Concept: A Present/Future situation of low probability/Or Zero probability

Ex: the 第一种，A present/future situation of low probability, I don't often see her.表明我明天不大可能会见到她。根据句式，就变成，if I saw her tomorrow, I would talk speak to her.

Ex: the 第二种，Zero probability, I don't have any time this week,表明本周没有时间了。根据句式，就成成，If I had some time, I would tidy up my office.

Third conditional Formulation

If + Past Perfect(had done) + would/wouldn't +Past participle(have done)

Concept: A Past situation=Too Late, Zero Probability

Ex: I didn't see her yesterday,就变成IfI had seen her, I would have spoken to her.

Ex: They weren't late last night,就变成  I wouldn't have let them in if they had been late.