 Let's start by having a look at the zero conditional. The zero conditional is used to talk about situations that are generally or always true, such as scientific facts. What's the result of heating ice? It melts. How can we put this in a sentence? A sentence could be, if you heat ice, it melts. Let's have a look at the structure of these sentences. The first part of the sentence, the first clause, if you heat ice, is using the tense present simple. The second clause is also using the tense present simple. And we have the if at the beginning of the sentence. If could be substituted by the word when to indicate the total certainty of the thing happening. Let's have a look at another example using the zero conditional with a slightly different situation. The result of me getting up early in the morning could be to go for a walk. That's a typical activity that I do. How could we put this into a sentence? This would be the typical sentence. If I get up early, I usually go for a walk. Again, we could substitute if with when to indicate the certainty of this happening. So the usages for the zero conditional are scientific facts, general truths, and issues or situations that are certain to happen or almost certain to happen. The use of when indicates that the situation is certain to happen.