The zero conditional is used to describe something that is always true. The two parts that make up the zero conditional sentence have the same tense: simple present.

if + present simple (conditional clause) present simple (main clause)
If I mix yellow with blue. I get green.
If you draw back the curtains. You let the sunlight in.
 

The positions of the two clauses above are not fixed. In other words, they can change places.

present simple if + present simple
I get green. If I mix yellow with blue.
You let the sunlight in. If you draw back the curtains.
 

When/whenever can be used instead of if.

Examples:

  • If/When I mix yellow with blue, I get green.

 

We can use the past simple instead of the present simple as above. The past simple must be used in both clauses.

Examples:

  • If he called her, she always said she was busy.
  • If I bet on a horse, the horse often came last.