Because conditional sentences are quite complex in both form and meaning, they are a problem for most learners of English. If you have a good understanding of the English tense system and of the modal auxiliaries, you will find it easier to understand and use conditional sentences. (The sentence you just read is a predictive conditional sentence.)
All conditional sentences contain a dependent clause and an independent clause. The dependent clause usually begins with if; it expresses a condition. The independent clause expresses a result of the condition. The if-clause is usually first, but the order of the clauses is usually not important. Thus, these two sentences have basically the same meaning:
If she goes to the store, she will buy ice cream.
She will buy ice cream if she goes to the store.
You have probably noticed that different teachers, textbooks, and Web sites sometimes explain the same thing in different ways. This seems to be especially true of conditional sentences. However, two different explanations can both be correct, especially if the difference is due to the fact that complicated material has been organized in different ways. This is often true of explanations of conditionals that you find in your textbooks. Here conditional sentences are divided into three types based on their meanings: real, predictive, and imaginative conditional sentences.
Also see The Sentence, Modal Auxiliaries, Verbs A-L, Verbs M-Z, Tenses, and Adverb Clauses.
A. Real conditional sentences can express generalizations and inferences.
1. Generalizations include facts that are always true and never change, and they include present or past habitual activities that are or were usually true.
Real conditionals expressing generalizations usually have the same tense (usually simple present or simple past) in both clauses. However, if the simple present tense is used in the if-clause, will + verb can be used in the main clause without changing the meaning.
Examples of real conditional sentences expressing facts:
If water boils, it turns to steam.
If water boils, it will turn to steam.
Examples of real conditional sentences expressing habitual activities:
If he eats breakfast, he feels better all day.
If he eats breakfast, he will feel better all day.
If he ate breakfast, he felt better all day.
These generalizations can also be expressed by using when or whenever instead of if:
When water boils, it turns to steam.
When he eats breakfast, he feels better all day.
When he ate breakfast, he felt better all day.
2. Inferences are often expressed in real conditional sentences.
Real conditionals expressing inferences usually have parallel verb phrases in both clauses. However, if a modal which explicitly expresses an inference (must or should, for example) is used in the main clause, parallel verb phrases are not used.
Examples of real conditional sentences expressing inferences:
If today is Wednesday, it is George’s birthday.
If I can do it, anyone can do it.
if it is raining, the streets are getting wet.
If he was at school, he saw the accident.
If today is Wednesday, it must be George’s birthday.
If I can do it, anyone must be able to do it.
if it is raining, the streets must be getting wet.
B. Predictive conditional sentences can express predictions and plans.
1. Predictive conditional sentences usually contain simple present tense in the if-clause and will or be going to in the result clause. However, a weaker modal of prediction (may or should, for example) can be used in the result clause to express less certainty.
2. Examples of predictive conditional sentences:
If the exam is hard, many students are going to fail.
If Mary does well on the final exam, she will get an A in the class.
If George does well on the final exam, he may get an A in the class.
If Fred studies, he should pass the exam.
C. Imaginative conditional sentences are the most difficult for many learners of English
because of the unusual relationship between form (the tenses used) and meaning.
In this type of conditional sentence, past tense refers to present or future time; past perfect tense refers to past time. Another problem for many learners of English is that were (not was) is used with singular subjects. Be is the only English verb with two past tense forms, but only one of them (were) is used in imaginative conditional sentences.
Imaginative conditional sentences can express hypothetical or contrary-to-fact events or states.
1. Hypothetical events or states are unlikely but possible in the present or future.
Imaginative conditional sentences expressing hypothetical events or states have a past tense verb in the if-clause and would + verb (or might or could + verb) in the result clause.
Examples of hypothetical conditional sentences (present and/or future time):
If George had enough money, he would buy a new car.
If I won the lottery, I would buy you a present.
If she knew the answer, she would tell us.
(George probably does not have enough money; I probably will not win the lottery; she probably does not know the answer.)
2. Contrary-to-fact events or states are either impossible in the present time or did not happen in the past.
Imaginative conditional sentences expressing present contrary-to-fact events or states have a past verb in the if-clause and would + verb (or might or could + verb) in the result clause. Some examples:
If I were you, I would not do that.
If she studied for exams, she would get better grades.
If it were raining, the streets would be wet.
(I am not you; she doesn’t study for exams; it isn’t raining.)
Imaginative conditional sentences expressing past contrary-to-fact events or states have a past perfect verb in the if-clause and would + have + verb (or might or could + have + verb) in the result clause. Some examples:
If George had had enough money, he would have bought a new car.
Source : http://faculty.deanza.edu/flemingjohn/stories/storyReader$18