Zero and first conditional

FORM AND USE

ZERO CONDITIONAL 

  We use the zero conditional to talk about things that are generally true or that always happen. 
In sentences like this, both verbs are in the present simple:
If you heat water to 100 degrees, it boils. 

We can change the order of the sentence: 
Water boils if you heat it to 100 degrees. 

Talking about facts 

We often use if sentences like this to talk about scientific facts: 
If you burn coal and other fossil fuels, this creates smoke. 
If the smoke mixes with water, it turns to acid rain. 
If the acid rain falls on trees, they die. 

Routines, systems and processes

We can use if sentences to talk about routines,systems and processes:
When David works, he listens to his favorite music. 
If the commute is long, I read a book on the train back home. 
James and Peter don't like shopping when they are on a holiday.

FIRST CONDITIONAL 

We can use if sentences to talk about the future. 

FORM

We use the present simple in the if part of the sentence and will + bare infinitive in the other part. 
If we sell these items at $3.65, we will make a good profit. 
We can change the order of the sentence: 
We will make a good profit if we sell these items at $3.65. 
Remember that we do not use will in the if part of the sentence: 
WRONG: If Enrico will come tomorrow, I will give him the contract. 
RIGHT: If Enrico comes tomorrow, I will give him the contract.

USE

In the if part of the sentence, we talk about real possibilities in the future. In the other part of the sentence, we talk about the result.
Jack's business colleague,Mr Suzuki, is coming tomorrow. At the moment Jack is ill at home. 
If Jack comes in tomorrow, he will look after Mr Suzuki. 
If Jack doesn't come in tomorrow, I will take Mr Suzuki to the factory. 

IF + IMPERATIVE

We can also use if sentences with imperatives to give instructions:
If he calls me this afternoon, ask him to call me/give him a message/tell him I want to see him.

IF AND WHEN 

There is an important difference between if and when: 
I will call you if the train is late. -> Maybe the train will be late, maybe not.
I will call you when the train gets in. -> I know the train will arrive. I am 100% sure.