THE FIRST CONDITIONAL SENTENCE

The first conditional has the present simple after 'if', then the future simple in the other clause:

If + sub + verb1 +(obj) +, + sub + will/shall + base form + (obj)

It's used to talk about things which might happen in the future. Of course, we can't know what will happen in the future, but this describes possible things, which could easily come true.

For example:

-If it rains, I won't go to the park.

-If I study today, I'll go to the party tonight.

-If I have enough money, I'll buy some new shoes.

-She'll be late if the train is delayed.

-She'll miss the bus if she doesn't leave soon.

-If I see her, I'll tell her.

First vs. Zero Conditional:

The first conditional describes a particular situation, whereas the zero conditional describes what happens in general.
For example (zero conditional):

-If you sit in the sun, you get burned (here I'm talking about every time a person sits in the sun - the burning is a natural consequence of the sitting)
But (first conditional):

-If you sit in the sun, you'll get burned (here I'm talking about what will happen today, another day might be different)

First vs. Second Conditional:

The first conditional describes things that I think are likely to happen in the future, whereas the second conditional talks about things that I don't think will really happen. It's subjective; it depends on my point of view.
For example (first conditional):

-If she studies harder, she'll pass the exam (I think it's possible she will study harder and so she'll pass)
But (second conditional):

-If she studied harder, she would pass the exam (I think that she won't study harder, or it's very unlikely, and so she won't pass).