We use the following structure to talk about actions which are very likely to take place in the future:
If it rains, I'll use my umbrella. (It rains sometimes).
If he goes to the station, he'll pick you up. (He can easily go to the station).
This type of conditional sentence is formed by IF + SUBJECT + PRESENT TENSE + FUTURE TENSE.
First conditional sentences are normally reversible. When the if-clause comes at the end, a comma is normally not required between the two clauses. The following forms mean exactly the same as their counterparts above.
I'll use my umbrella if it rains.
He'll pick you up if he goes to the station.
We use the first conditional for events that are very likely to happen in the future. In the examples above, it is very possible that it will rain or that he will go to the station. We can also use this structure with the imperative form.
This type of sentence is formed by IF + SUBJECT + PRESENT TENSE + IMPERATIVE CLAUSE.
If it rains, use your umbrella.
This form is also reversible and again a comma is not normally required between clauses: Use your umbrella if it rains.
We use when instead of if to say that the action or event is 100% certain to happen. These types of sentences are formed by WHEN + SUBJECT + PRESENT TENSE + FUTURE TENSE.
When I buy a new car, I'll give you my old one.
As with the forms above, this one is also reversible and a comma is not normally required: I'll give you my old car when I buy a new one.
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