Zero conditional: when and how to use

How to form

Structure of zero conditional
positive negative question
If/when you heat water to 100 degrees, it boils. If/when you don't heat water to 100 degrees, it doesn't boil. What happens if/when you heat water to 100 degrees?

When to use

We use the zero conditional to express that a particular condition always has the same result. We are saying that something is always true or is a fact:

[condition] [result]
If we don’t drink, . . . we get thirsty.
If you heat water, . . . it evaporates.
The zero conditional – common mistakes
Common mistakes Correct version Why?
they will get If people eat too much, they get fat.
We use the present simple in both clauses of a zero conditional. We are saying that the condition is always true.
when it will reach Water boils when it reaches 100°C.
If/when you don’t exercise, you get fat. If and when have the same meaning here. We can also use whenever.