A conditional clause expresses a condition – something which must happen first so that something else can happen. Conditional clauses are also called if-clauses because they are introduced by the subordinating conjunction if.
Zero conditional is used to talk about situations that always happen if something else happens. In zero conditional sentences, we use a simple present tense in both clauses.
- If I have guests, I always cook for them.
- If you pour oil on water, it floats.
- If you heat metals, they expand.
- If I am bored, I go for a walk in the woods.
- If I call he always comes.
- If she is angry, I leave her alone.
In most cases, it is possible to replace if by when.
- When you pour oil on water, it floats.