We use the so-called zero conditional when the result of the condition is always true, like a scientific fact.
Notice that we arethinking about a resultthat is always true forthis condition.
We are thinking about a simple fact. We use the present simple tense to talk about the condition.
We are talking about the future. We are thinking about a particular condition or situation in the future, and the result of this condition.
Notice that we arethinking about a futurecondition. It is not rainingyet. But the sky is cloudyand you think that itcould rain.
The second conditional is used for conditions unreal, impossible, improbable and refers to a hypothetical situation
if + simple past + simple conditional.If + Past Simple + modal + verb ↓ could might should would
If I won the lottery, I would travel around the world. (Si ganara la lotería, viajaría por todo el mundo).
It refers to a hypothetical situation of the past. would/could/might + If + Past Perfect tense + have + past participle
If she had come on Saturday, I would have seen her. (Si ella hubiese venido el sábado, la habría visto). If he had worked more, he could have saved more money. (Si él hubiese trabajado más, podría haber ahorrado más dinero).