Conditional clauses are used to express the idea that the truth of the proposition (what is said)  in the matrix clause depends on the fulfilment of the condition in the if-clause. Conditional clauses are often introduced by if. (go on reading, and everything will be clear.)

		Mr Smith is talking to his daughter. Which sentence do you think she would prefer to hear: 1 or 2?
		1- OK. I´ll buy you some sweets.
		2- I´ll buy you some sweets, if you stop crying.

You have chosen sentence number 1, haven't you? You have chosen it because the girl does not have to do anything to get the sweets. But, in sentence 2, she has to do something: she has to stop crying.

Sentence 2 has two parts:    a- I´ll buy you some sweets,      b- if you stop crying.
 
Part 'a' has a condition (part 'b') attached to it and the condition has to be fulfilled before the action in the matrix clause (part 'a') is carried out. That is why it is said that the truth of the proposition in the matrix clause depends on the fulfilment of the condition in the if-clause.

  It is impossible to say in advance whether or not Mr Smith will buy his daughter some sweets. We'll have to wait and see if she stops crying.

   The if-clause can go before or after the matrix clause: If you stop crying, I´ll buy you some sweets.  I'll buy you some sweets, if you stop crying.

There are three types of conditional clauses depending on whether the speaker thinks the condition:

  may be fullfilled,
  will not be fulfilled or it is highly unlikely that it will be fulfilled,
  can´t be fulfilled.

We know what the speaker thinks about the likelihood of the fulfillment of the condition through the tense he chooses to use in the if-clause.