Conditional sentences are sentences in which one thing depends upon another.
If I win the lottery then I will give all the money to charity.
We often leave out the word 'then'.
If I win the lottery I will give all the money to charity.
There are different types of conditional sentence.
1) Things that might actually happen
If it snows we will build a snowman.
This type of sentence uses the future tense ('we will').
2) Imaginary things or things that might possibly happen but are unlikely
If David Beckham came to our school I would be very excited.
This type of sentence uses words like 'could' and 'would'.
3) Things that cannot be changed because they are in the past
If I had worked harder I would have passed the test.
This type of sentences uses words like 'could have' and 'would have'.