Zero Conditional is used to express a general truth the probable future .

In the if/when-clause we usually use Present Simple Future Simple .

In the main clause we usually use Present Simple Future Simple .

In this type of conditionals, we can cannot use "when" instead of "if".

"Unless" means "if" "if not" .

When the main clause comes before the if-clause, we use do not use a comma between them.

When the if-clause comes before the main clause, we use do not use a comma between them.