Zero Conditional Sentences 

In zero conditional sentences, the tense in both parts of the sentence is the Present Simple Tense. These conditionals sentences are used when the result will always happen. They often refer to general truths, such as scientific facts; e.g.

1. If you freeze water, it becomes a solid.
2. If people eat too much, they get fat.
3. If water reaches 100 degrees, it always boils.
4. Plants die if they don’t get enough water.
5. If my son has a cold, I usually catch it.
6. If public transport is efficient, people stop using their cars.
7. People die if they don’t eat.
8. You get water if you mix hydrogen and oxygen.
9. Snakes bite if they are scared.
10. If babies are hungry, they cry

NOTE-I: In zero conditional sentences, you can replace IF with WHEN because both express general truths. The meaning will be unchanged; e.g.

If you heat ice, it melts.
= When you heat ice it melts.

NOTE-II: The zero conditional is also often used to give instructions, using the imperative in the main clause; e.g.

1. If Rohit phones, tell him to meet me at the restaurant.
2. If you want to come, call me before 5:00.
3. Meet me here if we get separated.

Comparison — Zero Conditionals and Type-I Conditionals

We use the Type Conditional Sentences, the situation described in the main clause is imaginary, and that imaginary situation is quite likely to happen if the condition described in the IF-CLAUSE is fulfilled. But in the zero conditional sentences there is no imagination at all, if the condition described in the IF-CLAUSE is fulfilled, the result will always happen. Now compare these sentences.

1. If she runs she’ll reach the station in time.

[This is a type-I conditional sentence, not the zero conditional as the situation of the main clause i.e. REACHING THE STATION IN TIME might not always happen, sometimes it’s also possible even if she runs she is late to reach the station.]

2. If you freeze water, it becomes a solid.

This is a zero conditional sentence, not the type-I conditional as the situation of the main clause i.e. CHANGE OF WATER INTO SOLID is a fact and will always happen if someone freezes it. There is no element of imagination (unreality) involved in it.]

NOTE: You can’t replace IF of type-I conditional sentences with WHEN, whereas in zero conditional sentences IF and WHEN are replaceable.