First Conditional: real possibility
We are talking about the future. We are thinking about a particular condition or situation in the future, and the result of this condition. There is a real possibility that this condition will happen. For example, it is morning. You are at home. You plan to play tennis this afternoon. But there are some clouds in the sky. Imagine that it rains. What will you do?
IF condition result
present simple WILL + base verb
If it rains I will stay at home.
Notice that we are thinking about a future condition. It is not raining yet. But the sky is cloudy and you think that it could rain. We use the present simple tense to talk about the possible future condition. We use WILL + base verb to talk about the possible future result. The important thing about the first conditional is that there is a real possibility that the condition will happen. Here are some more examples (do you remember the two basic structures: [IF condition result] and [result IF condition]?):
IF condition result
present simple WILL + base verb
If I see Mary I will tell her.
If Tara is free tomorrow he will invite her.
If they do not pass their exam their teacher will be sad.
If it rains tomorrow will you stay at home?
If it rains tomorrow what will you do?
result IF condition
WILL + base verb present simple
I will tell Mary if I see her.
He will invite Tara if she is free tomorrow.
Their teacher will be sad if they do not pass their exam.
Will you stay at home if it rains tomorrow?
What will you do if it rains tomorrow?
Sometimes, we use shall, can, or may instead of will, for example: If you are good today, you can watch TV tonight.