There are four types of conditional sentences ：  Real present  Real future  Unreal present / future  Unreal past

When referring to conditional sentences, we will refer to the if clause and the result clause, that is the main clause of sentence. I would have met you at the airport if I had known you were coming. RESULT CLAUSE IF CLAUSE

The order of the if clause and the result clause is interchangeable. If the result clause is first, there is no comma between the clauses ; but if the if clause if first, a comma comes before the result clause. If I had known you were coming, I would have met you at the airport. RESULT CLAUSE IF CLAUSE

Real conditionals are used when the if clause expresses an idea that is probably true, or at least very possible. A real present condition is one in which the situation is true in the habitual present time. This form is usually used to express general truths. If it rains, we swim at the gym. Water boils if it is heated to 212 degrees Fahrenheit.

In the real present condition ( like in the above examples ), a present tense used in both the if clause and in the result clause. In these examples, the simple present is used, but depending on the intended meaning, we could also see the present progressive, a present modal, or even the present perfect, as the following examples illustrate. If it is raining, we may swim at the gym. Water boils if it has been heated to 212 degrees Fahrenheit.

A real future condition is one in which the situation will most probably be true in the future. If it rains tomorrow, we will carry our umbrellas. In a real future condition, we use a present tense verb in the if clause, and a future tense (any future tense) in the result clause. Imperative verb forms are also possible in the result clause. If the sun is shining tomorrow, he will have already gone to the beach by the time we get to his house. Tell me if you see anything strange.

Real future conditions can also be formed with the modal should. This conveys the impression that the action in the if clause is a little less likely ( though still far more likely than it would be if the unreal present or future were used ). It is also fairly formal. If I see her, I will tell her. ( It is likely that I will see her. ) If I should see her, I will tell her. (It is a little less likely that I will see her. ) Note that the modal will is almost never used in an if clause.

We use unreal conditions when the idea expressed in the if clause is impossible or unlikely. The unreal / future condition is one in which the action is impossible or highly doubtful in the opinion of the speaker. If I called the president, he probably wouldn't’t speak to me. If the United States won the next World Cup Games, the entire world be shocked.

The time reference in this type if conditional is either to a repeated, habitual event in the school or to a specific point in the future : If I called the resident, he probably wouldn't’t speak to me. Note that the verb in the if clause is identical in form of past and the verb form in the result clause begins with the modal would plus the simple form of the main verb. In reality, either the simple past or the past progressive could be used in the if clause. And would, could, or might could be used in the result clause.

If + simple past/past progressive,subject+would/might/could+simple word form of the verb If the United States won the next World Cup Games,the entire world would be shocked. If they were playing instead of sleeping,their mother might be very angry. A past tense verb in an If clause refers to the unreal present and not the past. Progressive forms are possible in the result clause: If their mother were working,the children would probably be playing.

*Pay attention to the verb be. In the unreal present/future condition, it always appears in the form were in the if clause. Formal:If he were here,I would tell him exactly how I feel. Formal:I would sell my house right now if it were valued at $ Informal:If I was rich,I would buy a new car. Were+infinitive can replace an unreal present verb: If I were to call the president,he probably wouldn’t speak to me.

Unreal Past Conditional Sentences An unreal past conditional is one in which the situation did not occur in the past.In such sentence, the speaker is imagining the past as different form way it happened. The speaker is talking about “the way things could have been” under a different set of condition. If there had been more time, we would have finished the project. Lisa would never have seen South America if she hadn’t met that young man from Bolivia.

In the unreal past, the verb in the if clause is identical in form to the past perfect(or past perfect progressive) and the verb in the result clause contains the modal would, might, or could plus the perfect auxiliary have plus the past participle of the main verb. If +past perfect or past perfect progressive, subject+would/might/could+HAVE+past participle If she hadn’t been dancing, she would never have broken her leg. If we had seen you at the party, we might have stayed longer. Like the unreal present, the result clause may contain a progressive form: If I had taken that job that paid so little, I would have been watching every penny I spent.

It is possible to mix certain conditions. It is quite common, to mix an unreal past if clause and an unreal present result clause. If you had been born in Japan, you would speak Japanese. If I hadn’t gotten into that car accident last week, I would be swimming in the Caribbean right now.

Sometimes the word if can be omitted form a condition sentence. If can be omitted:  In an unreal past condition  In an unreal present condition if the main verb or auxiliary verb is 0 (were)  In a real future condition that contains the modal should

If the word if is omitted, the subject and the auxiliary are reversed, and the sentence begins with a verb. The omitted form appears more in writing than in speech and is considered to be rather formal. If he had called me, I would not have been so angry. Had he called me, I would not have been so angry.

If it were sunny today,I would gladly volunteer to go to the rally. Were it sunny today,I would gladly volunteer to go the rally. If you should need help,don’t hesitate to call. Should you need help,don’t hesitate to call If the word if is omitted from a clause in the negative,the word not is separated from the auxiliary and placed before the main verb.In this case,it cannot be contracted.In each of the below pairs,the second examples are also considered formal and are rarely spoken.

If she hadn’t called me,I would never have heard the news. Had she not called me, I would never have head the news. If it weren’t such a long drive,I would have gone. Were it not such a long drive,I would have gone.