Zero conditional (if)

Introduction

We use the zero conditional to describe the result of an action when the result is always or usually the same, for example:

general facts or truths
habits or routines
to give instructions or make suggestions

Examples

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.

Form

The zero conditional has two clauses:
the conditional clause and the main clause.

Conditional clause

This clause describes the action or condition that makes the main clause true.

We can use "when" or "if":

"when": it will happen
"When I go on holiday ..." (I will have a holiday – I am sure.)
"if": it might happen
"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:

Present simple
"When I go on holiday, I go somewhere warm."
present continuous
"If I’m reading a book, I wear glasses."
Present perfect
"When I’ve finished work, I go to the gym."

Main clause

This clause describes the result of the conditional clause. We use the present simple and the imperative:

Present simple:
- 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.
imperative:
- for suggestions or instructions
  "If you have time, come to my house."
  "When you get to the main road, turn right."

Order

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."