If and when are both used to introduce conditional clauses*. Most often, we use them interchangeably with no specific grammar rule in mind. Consider the following sentences:
1. I won’t be able to come when it rains.
2. My father will buy me a car if I am twenty.
Do you think the above sentences are correct regards the use of if and when? Well, you’ll soon find out whether they are or not.
Rule: Use when for conditions you are certain of, that is, you are certain the incidence will happen. For if, use it for conditions that are possible, but not certain. Let’s now analyze the above sentences.
Sentence 1. I won’t be able to come when it rains: when shouldn’t have been used here, but rather if because one cannot be certain it will rain. So, the sentence would rightly be: I won’t be able to come if it rains.[ It may rain; and should that happen, I will not come. But if it doesn’t, I will. I am not certain]
Sentence 2.My father will buy me a car if I am twenty: The conditional clause, [if] I am twenty, here is certain. Age keeps increasing so one will, without doubt, grow and be twenty. As a result, if cannot be used but rather when. The sentence would rightly be: My father would buy me a car when I am twenty.[ I will turn twenty one day and when I do, my father will by me a car. I am certain of it.]
More examples:
1. If you see him, tell him I want to have a word with him.( not when). [ There is a probability you won’t see him. Assuming you do, tell him…….I am not completely sure he will be seen]
2. I am going to London; I will phone you when I get back.( not if) [ I am certain that I will get back, and once I do, I will phone you]
3. If it rains tomorrow, we won’t have a picnic. (not when). [ It may rain tomorrow and in case it does, …… I am not certain it will rain, just assuming.]
4. Dennis called to tell me he is coming; when he comes, ask him to wait for me. [ I am certain he will come, so when is used instead of if]
In conclution, use when for future events that are certain to happen. And use if for future events that may happen, but you are not completely certain of it.
*Conditional clauses are dependent clauses used to express the condition under which the action in the independent/main clause can be fulfilled.
Grammar Rules 