-we can make a zero conditional sentence with two present simple verbs (one in the 'if clause' and one in the 'main clause'):
If + present simple, .... present simple.
-this conditional is used when the result will always happen
-so, if water reaches 100 degrees,it always boils.
-it's a fact.
-I'm talking in general, not about one particular situation.
-the result of the 'if clause' is always the main clause.
-the 'if' in this conditional can usually be replaced by 'when' without changing the meaning.




-the first conditional has the present simple after 'if', then the future simple in the other clause:
If + present simple, .... will + infinitive.
If it rains, I won't go to the park.
-it's used to talk about things which might happen in the future
-of course, we can't know what will happen in the future, but this describes possible things, which could easily come true.



-the second conditional uses the past simple after if, then 'would' and the infinitive:
If + past simple, ...would + infinitive.
-we can use 'were' instead of 'was' with 'I' and 'he/she/it', this is mostly done in formal writing

It has two uses:

First we can use it to talk about things in the future that are probably not going to be true
-maybe I'm imagining some dream for example
If I won the lottery, I would buy a big house (I probably won't win the lottery)

Second we can use it to talk about something in the present which is impossible, because it's not true
If I had his number, I would call him. (I don't have his number now, so it's impossible for me to call him).