conditional sentence: the introduction

Here are some of the most common uses of conditional sentences:

1. Uncertain events & situations

We use conditional sentences to talk about uncertain events and situations. 
Example:
Ask Mark if he is staying tonight. (He may or may not be staying)
If I see Nicky, I will convey your message. (I may or may not see Nicky) 

 

2. Conditions

An if-clause can also refer to a condition – something which must happen first, so something else can happen.
Example:
If we can get there by tomorrow morning, we can have breakfast at Mark’s place.
I will go to market tomorrow if I am able to repair my car today.

 

3. If…then

We sometimes construct sentences with if…then to emphasize that one thing depends on another. But note that we do not use if…so in this way.
Example:
If he cannot walk, then she will have to help him. (NOT …so we will have to go and see her)
If Jennifer has no enough money, then we have to lend her some money.

 

4. If meaning “if it is true”

Another common use of if is to mean “if it is true that” or if it is the case that”
Example:
If you were in New York, why didn’t you come and see us?
If it will help you, I will lend you some money 

 

5. Unlikely, imaginary or untrue situations

Past (or special) tenses can be used to give the idea that something is unlikely, imaginary or untrue.
Example:
If I married you, we would both be unhappy. (Past tense used to talk about an imaginary future situation).

 

Position of if clauses

Note that an if-clause can come at the beginning or end of a sentence. When an if-clause comes first, it is often separated by comma. Compare:

If you eat too much, you get fat.
You get fat if you eat too much. 

 

Zero, first, second & third conditionals

Most student grammars focus on four common patters with if which are often called zero, first, second and third conditionals as given below:

Zero conditional (used to talk about the general or scientific conditions which are always true)

If + present - infinitive

E.g. If you heat water, it boils. 

First conditional (used to talk about the future consequences of a specific event or situations)

If + present - will + infinitive

E.g. If we play tennis I will win. 

Second conditional (used to talk about unlikely, imaginary or untrue events or situations)

If + past – would + infinitive

E.g. If we played tennis, I would win. 

Third conditional (used to talk about things that didn't happened in the past and the consequence if they had happened)

If + past perfect – would have + past participle

E.g. If we had played tennis, I would have won.

taken from: http://www.conditionalsentences.org/