Look at the first conditional. Notice the different clauses.
( if + Present Simple)
(will/won't + infinitive)
If I start teaching again,
I'll be exhausted after a year
· The first conditional talks about the result of a possible event or situation in the future.
· The if clause talks about things that are possible, but not certain: If I start teaching again, I'll be exhausted after a year (maybe I will start teaching again).
The main clause says that we think the result will be in this situation (I'm sure I will be exhausted after a year).
· We make the first conditional with: if+ Present Simple, will/won't + infinitive
· The if clause can be first or second in the sentence: I'll be exhausted after a year if I start teaching again.
· We often use might in the main clause to mean ' will perhaps': But you might have to wait until next year if you don't apply soon.
· We can use unless to mean 'if not' in the first conditional: Unless I do it now, I'll be too old. = If I don't do it now, I'll be too old.
Zero conditional; conditionals with modal verbs and imperatives, in case
· The zero conditional talks about things that are always true :
If you have children, you worry about them all the time.
· In the zero conditional both verbs are in the Present Simple: If children stay indoors all the time, they become unfit.
*If and when have the same meaning in the zero conditional: If/When I'm worried, I don't sleep very well.
Zero or first conditional
Compare these sentences:
If the children aren't home by five, I call the school.
This sentence is a zero conditional. It talks about something that is always true.
If the children aren't home by five, I'll call the school.
This sentence is a first conditional. It talks about one specific time in the future.