In English, we use conditional sentences for events or occurrences that are more or less certain under particular circumstances. Often, these employ the word "if" in the first clause, and then follow with a main clause. There are four basic types of conditional sentences that describe levels of possibility, from events that are very likely to missed opportunities in the past.  

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Type 0 conditional sentences state facts or universal truths. The "if" clause and the main clause simply use the present simple tense.

If you are in the Skycouch row, there are special seat belt instructions in your seat pocket.

Caption 11, Air New Zealand - An Unexpected Briefing

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Type 1 conditional sentences refer to cause-and-effect links, and events that are quite certain or even definite if the condition stated in the "if" clause is fulfilled. The "if" clause is formed with "if" + simple present tense, and the main clause is uses the "will" future. 

So, if you observe these writing rules, your letter will be easy to read,​

Caption 12, Business English - Cover Letter

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If they are too late, they will miss their ride.

Caption 26, Nature & Wildlife - Wild Sharks

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Type 2 conditional sentences refer to events that are less possible or likely, often hypothetical. The "if" clause uses the simple past, which actually creates the subjunctive mood, while the main clause contains "would" + the infinitive (together sometimes referred to as conditional I tense).

If you gave me a chance, I would take it

Caption 14, Clean Bandit - Rather Be (feat. Jess Glynne)

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If I had the vocal capacity, I would sing this from every mountain top.

Caption 37, Jamila Lyiscot's TED talk - 3 Ways to Speak English

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Type 3 conditional sentences are used to talk about possibilities or events that never came to be. The "if" clause contains the past perfect, while the main clause includes "would have" + past participle (sometimes in combination referred to as the conditional II tense). 

Unfortunately, if we had signed the contract last week,

we would have been able to make some concessions.

Captions 24-25, Business English - Difficulties with Coworkers and Contracts

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It is worth mentioning that you may often see "mixed types" of the conditional, in which a missed opportunity in the past (expressed using the participle) is portrayed as still affecting the present. Take a look at the following sentence. It is clear that Chuck did not crash his motorcycle, yet the main clause is still being expressed as if it were part of a type two conditional sentence.

If Chuck had crashed it, we would be out.

Caption 65, Motorcycle Masters - Birmingham Alabama

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Further Learning
Whenever you see a sentences with "if" on Yabla English, try to identify which type of conditional sentence it might be related to. Make up 3 or 4 sentences related to your plans for the week or anything you didn't get to do over the weekend. For example, "If Anna had wanted to go to the cinema, I would have gone with her," or "If I can get the afternoon off tomorrow, I will go to the cinema."