conditional if

Conditionals

A conditional sentence is a sentence containing the word if. There are three common types of conditional sentence:

1. if clause > present simple tense : main clause > future tense (will)
   • If you help me, I will help you.
   • If I win the lottery, I will buy a new car.
   • If it snows tomorrow, we will go skiing.
2. if clause > past simple tense : main clause > would
   • If you knew her, you would agree with me.
   • If I won the lottery, I would buy a new car.
   • If it snowed tomorrow, we would go skiing.
3. if clause > past perfect tense : main clause > would have
   • If you had helped me, I would have helped you.
   • If I had won the lottery, I would have bought a new car.
   • If it had snowed yesterday, we would have gone skiing.

Of course, it is possible to start conditional sentences with the main clause:

• I will buy a new car if I win the lottery.
• I would buy a new car if I won the lottery.
• I would have bought a new car if I had won the lottery.

English speakers choose one of the three conditional structures as follows:

1. Conditional one - to express a simple statement of fact or intent
   
   
   • I will buy a new car if I win the lottery.
   • I will go home if you don't stop criticizing me.
   • You will fail your exams if you don't start working harder.
   • She will lose all her friends if she continues to talk about them behind their backs.
2. Conditional two - to refer to a present unreal situation or to a situation in the future that the speaker thinks is unlikely to happen
   
   
   • If I had a lot of money, I would buy a new car. (but I don't have a lot of money)
   • If I were you, I would tell him you're sorry. (but I am not you)
   • If I won the lottery, I would buy a new house. (but I don't expect to win the lottery)
   • If it snowed tomorrow, we would go skiing. (but I don't have much hope that it will snow)
3. Conditional three - to refer to the past and situations that did not happen
   
   
   • If it had snowed yesterday, we would have gone skiing. (but it didn't snow, so we didn't go skiing)
   • If you had studied harder, you would have passed your test. (but you didn't study hard, so you didn't pass your test)
   • If I had known that, I would have told you. (but I didn't know, so I didn't tell you)
   • If she hadn't been driving slowly, she would have had an accident. (but she was driving slowly, so she didn't have an accident)

* Note: The way native speakers of English express conditions (use if-clauses) is much more varied than the 3 rigid combinations of tenses exemplified on this page. Learners should consult a good grammar reference work for a deeper understanding of this complex aspect of English grammar.

sumber : http://esl.fis.edu/grammar/rules/cond.htm