In zero conditional sentences, the tense in both parts of the sentence is the simple present. This conditional is used when the result will always happen. So, if water reaches 100 degrees, it always boils. It's a fact. I'm talking in general, not about one particular situation. The result of the 'if clause' is always the main clause.
The 'if' in this conditional can usually be replaced by 'when' without changing the meaning.
For example: If water reaches 100 degrees, it boils. (It is always true, there can't be a different result sometimes). If I eat peanuts, I am sick. (This is true only for me, maybe, not for everyone, but it's still true that I'm sick every time I eat peanuts)
|If clause (condition)||Main clause (result)|
|If + simple present||simple present|
|If this thing happens||that thing happens.|
As in all conditional sentences, the order of the clauses is not fixed. You may have to rearrange the pronouns and adjust punctuation when you change the order of the clauses, but the meaning is identical. In zero conditional sentences, you can replace "if" with "when", because both express general truths. The meaning will be unchanged.
- If you heat ice, it melts.
- Ice melts if you heat it.
- When you heat ice, it melts.
- Ice melts when you heat it.
- If it rains, the grass gets wet.
- The grass gets wet if it rains.
- When it rains, the grass gets wet.
- The grass gets wet when it rains.
The zero conditional is used to make statements about the real world, and often refers to general truths, such as scientific facts. In these sentences, the time is now or always and the situation is real and possible.
- If you freeze water, it becomes a solid.
- Plants die if they don't get enough water.
- If my husband has a cold, I usually catch it.
- If public transport is efficient, people stop using their cars.
- If you mix red and blue, you get purple.
The zero conditional is also often used to give instructions, using the imperative in the main clause.
- If Bill phones, tell him to meet me at the cinema.
- Ask Pete if you're not sure what to do.
- If you want to come, call me before 5:00.
- Meet me here if we get separated.