A conditional sentence is one in which contains a condition, an if-then situation. What will happen depends on something else. There are 4 kinds of contional sentences- zero conditional, first conditional, second conditional and third conditional.
The Zero Conditional
The zero conditional is a structure used for talking about general truths, or scientific facts – things which always happen under certain conditions.
A zero conditional sentence consists of two clauses, an “if” clause and a main clause. The simple present tense is used in both clauses. Note that ‘when’ can be used instead of ‘if’ in these types of sentences.
If you freeze water it turns to ice.
The first conditional is a structure used for talking about possibilities in the present or in the future.
If Abbey doesn’t get the job in San Francisco, she will marry her boyfriend.
The second conditional is a structure used for talking about unreal situations in the present or in the future. The action is characterized by unreality.
If Oscar and Olivia had the money, they would take a cruise around the world.
The third conditional is a structure used for talking about unreal situations in the past. In other words, it is used to talk about things which DID NOT HAPPEN in the past. The action is characterized by impossibility.
If Mike had known his new job required so much travel, he would not have taken it.
For more information about conditionals takes a look at
An explanation of conditional sentences: zero, first, second and third conditional– www.understandinggrammar.com