We use the zero conditional to describe the result of an action when the result is always or usually the same, for example:
to give instructions or make suggestions
If you add salt to water, it boils at a higher temperature. I listen to music when I travel by train. If you go to London, use public transport instead of taxis.
The zero conditional has two clauses:
the conditional clause and the main clause.
This clause describes the action or condition that makes the main clause true.
We can use "when" or "if":
"When I go on holiday ..." (I will have a holiday – I am sure.)
"If I go on holiday ..." (I might have a holiday – I'm not sure.)
We can use any form of present tense in English in the conditional clause, for example:
"When I go on holiday, I go somewhere warm."
"If I’m reading a book, I wear glasses."
"When I’ve finished work, I go to the gym."
This clause describes the result of the conditional clause. We use the present simple and the imperative:
for a general fact, habit or routine
"If I take the train, I buy a newspaper."
We can use adverbs to describe how usual the result is:
"If I take the train, I often buy a newspaper.
for suggestions or instructions
"If you have time, come to my house."
"When you get to the main road, turn right."
The order of the clauses does not matter: the meaning is the same.
If we put the conditional clause before the main clause, we use a comma to separate the clauses:
"When" / "If" + present tense, present simple
"When Sarah goes to London, she doesn’t take her car."
We don't use a comma if we put the main clause before the conditional clause:
present simple + "when" / "if" + present tense
"Sarah doesn’t take her car when she goes to London."