Conditional sentences

Learners of English often have problems with conditional sentences. One problem is knowing which conditional to use. We may then have difficulties with the construction of the conditional sentence.
Here are all four conditional forms in English, with examples and explanations of when and how to use them.
Zero conditional
If people eat too, much they get fat.
Zero conditional – examples, structure, usage

First conditional
If I have time, I will visit you.
First conditional – examples, structure, usage
Second conditional
If I had more time, I would travel the world.
Second conditional – examples, structure, usage
Third conditional
If I had had time, I would have gone to the football match.
Third conditional – examples, structure, usage
See also: Stuart’s article Common mistakes in conditional sentences.