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Conditional Sentence

In English conditional sentences, the condition clause (protasis) is most commonly introduced by the conjunction if, or sometimes other conjunctions or expressions such as unless, provided (that), providing (that) and as long as. Certain condition clauses can also be formulated using inversion without any conjunction (should you fail...; were he to die...; had they helped us...).
In English language teaching, conditional sentences are often classified under the headings zero conditional, first conditional (or conditional I), second conditional (or conditional II), third conditional (or conditional III) and mixed conditional, according to the grammatical pattern followed.[3]
"Zero conditional" refers to conditional sentences that express a simple implication (see above section), particularly when both clauses are in the present tense: "If you don't eat for a long time, you become hungry." This form of the conditional expresses the idea that a universally known fact is being described: "If you touch a flame, you burn yourself." The act of burning oneself only happens on the condition of the first clause being completed. However such sentences can be formulated with a variety of tenses (and moods), as appropriate to the situation.

More examples of the zero conditional
If you exercise a lot, you get fit
If you don't eat or drink, you die
If you eat lots of junk food, you will get fat

"First conditional" refers to predictive conditional sentences (see above section); here, normally, the condition is expressed using the present tense and the consequence using the future: "If you make a mistake, someone will let you know."
"Second conditional" refers to the pattern where the condition clause is in the past tense, and the consequence in conditional mood (using would or, in the first person and rarely, should). This is used for hypothetical, counterfactual situations in a present or future time frame (where the condition expressed is known to be false or is presented as unlikely).
If I liked parties, I would attend more of them.
If it rained tomorrow, I would dance in the street.
The past tense used in the condition clause is historically the past subjunctive; however in modern English this is identical to the past indicative except in the case of the verb be (first and third person singular), where the indicative is was and the subjunctive were. In this case either form may be used (was is more colloquial, and were more formal, although the phrase if I were you is common in colloquial language too):
If I (he, she, it) was/were rich, there would be plenty of money available for this project.
"Third conditional" is the pattern where the condition clause is in the past perfect, and the consequence is expressed using the conditional perfect. This is used to refer to hypothetical, counterfactual (or believed likely to be counterfactual) situations in the past
If you had called me, I would have come.
"Mixed conditional" usually refers to a mixture of the second and third conditionals (the counterfactual patterns). Here either the condition or the consequence, but not both, has a past time reference:
If you had done your job properly, we wouldn't be in this mess now.
If we were soldiers, we wouldn't have done it like that.