if + subject + present tense, subject + modal + infinitive
If I have time, I'll do it this afternoon.
Use of the first conditional
The first conditional is used to talk about the future.
We use it to describe situations that may really happen:
If I see Michaela, I'll tell her to call you.
(It is possible you will see Michaela.)
If it rains, we'll have to eat indoors.
(It is possible that it will rain.)
The first conditional is not used for imaginary situations :
If I had time, I'd travel more.
Nor is it used to talk about the past :
If I'd known you wanted to go, I'd have asked you.
Changing the position of the clauses; punctuation
The position of the two clauses can be changed without changing the meaning of the sentence :
If I have time, I'll give you a call.
This has exactly the same meaning as :
I'll give you a call if I have time.
Punctuation Note that when the if clause comes first, it is followed by a comma; when the result clause comes first, there is no comma.
When / as soon as
A similar structure can be made using when or as soon as instead of if
I'll tell her when I see her.
Compare with :
I'll tell her if I see her.
In the first example, the speaker is sure that he will see her.
In the second example, the speaker may see her or he may not.
I'll call you when I get to the airport.
I'll call you as soon as I get to the airport.
These two sentences are basically similar. As soon as emphasizes that you will do the action in the result clause immediately after the action in the conditional clause.