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