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?

The Zero Conditional

We can make a zero conditional sentence with two present simple verbs (one in the 'if clause' and one in the 'main clause'):

If + present simple, .... present simple.

This conditional is used when the result will always happen. So, if water reaches 100 degrees, it always boils. It's a fact. I'm talking in general, not about one particular situation. The result of the 'if clause' is always the main cluase. The 'if' in this conditional can usually be replaced by 'when' without changing the meaning.
For example: If water reaches 100 degrees, it boils. (It is always true, there can't be a different result sometimes). If I eat peanuts, I am sick. (This is true only for me, maybe, not for everyone, but it's still true that I'm sick every time I eat peanuts)
Here are some more examples: