There are four basic conditional sentence patterns where our choice of tense depends on the time of the condition (past, present or future) and how possible or impossible we think the event is:
 

Zero conditional
Possible at any time, but most commonly in the present:
If your dog is depressed, he probably needs more exercise.

If I eat dairy products, I get red spots on my skin.

Prawns are very risky to eat if they haven’t been kept at the right temperature.
If she’s travelling abroad on business, she always phones me every evening.
 

First conditional
Possible in the future:
If the single flops, they'll sack their manager.

If the results of the customer survey are favourable, the supermarket is going to introduce a new range.

If you’re coming on the motorway, you’ll need change for the tolls.
 

Second conditional
Impossible in the present/possible (but improbable) in the future:
If you were a bit nicer, he wouldn't get so cross.

The blockades wouldn’t happen if the police were firmer with the strikers.

If a celebrity were staying in the hotel, security arrangements would be tightened.

 
Third conditional
Impossible in the past:
If you had answered the door, she wouldn't have gone away again.

If someone had been teasing your child so nastily, you would have behaved in the same way.
It might have been easier to break the news if I had known her a bit better.
If the spy had intercepted the message, he could have averted the crisis.