Conditional Sentences

What is conditional sentences?
A conditional sentences consists of two clause, they are the if-clause and the main clause. The if-clause can come first or second. When it comes first, we usually put a comma between the two clauses.
Examples:

• If I have enough money, I will go to Hongkong.
• I will go to Hongkong, If I have enough money.

There are 3 types of conditional sentences and mixed conditional

• Conditional Sentences Type 1

The if-clause is in the present tense, the main clause uses will and the infinitive, or simple present.

Type 1: If Clause                           Main Clause                                 Meaning

              Simple Present            Simple Present            True in the present or possible

              If you work hard,            You Succeed.                                in future

                                                    Simple Future           It's possible to happen in the future

                                                  You will succeed.

And when do we use conditional sentences type 1?

a.  We use conditional sentences type 1 to talk about possible situations in the present or future.

     - If you leave earlier, you will not be late.

     - If you open the windows, I will come to your party.

b.  We often use conditional type 1 to talk about facts or processes.

     - If you heat water to 100 degrees, it will boil.

     - If we stare into the sun, we will hurt our eyes.

*Note: Other modal verbs can also be used in place of will and would

- If it rains like this all day, the river might flood. (might = will possibly)
- If it rains like this all day, the river could flood. (could = will be able to)

• Conditional Sentences Type 2

The if-clause is in the simple past or the past continuous tense. The main clause uses would and the infinitive, or would be and the present participle (verb-ing).
Type 2:       If Clause                     Main Clause                               Meaning
                 Simple Past           Would + Simple Form        Untrue in the present
            If you worked hard,    You would succeed.            Fact: You don't work hard,
             Past Continuous      Would be + Present             so you don't succeed.
           If it were not raining         Participle                       Fact: It's raining now, so I'm
                    now,                 I would be going out for           not going out for a walk.
                                                        a walk.

And when do we use conditional sentences type 2?
Conditional sentences type 2 is used to talk about actions or situations that are not talking place in the present or future, but we can imagine the probable result.
- If I were you, I would accept him. (In fact, I'm not you, so I will not accept him)
- If he were here, I would tell him about my plan. (In fact, he isn't here)

*Note: - Were' is used for both singular and plural subjects.
            -  The use of type 2 conditional in "If I were you, I would......." is a common form of advice.

• Conditional Sentences Type 3

The if-clause is in the past perfect or the past perfect continuous tense, the main clause uses would have and past participle (verb 3), or would have been and present participle (verb-ing).
Type 3:          If Clause                          Main Clause                          Meaning
                    Past Perfect                 Would have + Past             Untrue in the past
              If you had worked hard,           Participle                 Fact: You didn't work hard,
                    Past Perfect                  You would have                so you didn't succeed.
                    Continuous                       Succeeded.                Fact: It was raining yesterday
                 If it had not been             Would have been +       afternoon. I was not going out
                raining yesterday              Present Participle                     for a walk.
                     afternoon,                 I would have been going
                                                              out for a walk.

And when do we use conditional sentences type 3?
Conditional sentences type 3 is used to talk abut actions or situations that did not take or were not taking place in the past,  but we can imagine the probable result.
- If he had not been late this morninf, his teacher would not have punished him. (In truth, he was late)
- If I had studied hard, I would have passed the exam. (In fact, I didn't study hard, so i would not pass the exam).




Sources:

http://www.misterguru.web.id/2011/09/conditional-sentences-type-1-type-2.html

http://grammar.ccc.commnet.edu/grammar/conditional2.htm