Zero Conditional Sentences
The zero conditional is used to refer to conditions which are always true. So, if water reaches 100 degrees, it always boils. It’s a fact. We are talking in general, not about one particular situation.
In zero conditional sentences, the word “if” can usually be replaced by the word “when” without changing the meaning.
The result is always fulfilled when the condition which is given in the if-clause happens.
The if-clause can be at the beginning or at the end of a sentence.
Because of the certainty you can replace “if” with “when”