If I Were You...

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Although the second and the third conditionals in English are both used to talk about things that we think are unreal, they deal with two very different scenarios. The former appears in situations where we think the condition (which is expressed by the part of the sentence containing the conjunction 'if' or 'Unless') is counterfactual or rather unlikely to happen. The latter, on its part, is about the past and it denotes a condition that we think had the possibility of happening sometime in the past though it didn't really happen. 
 
If he worked hard, he would pass the exam. (Though we admit that he has the chance to pass the exam with hard work, we don't really think he'll work hard.) 

If he had worked hard, he would have passed the exam. (Though we admit that it he had the chance to pass the exam with hard work, he didn't work hard and therefore didn't pass the exam.)

However, when the condition is a thing of the past but the result concerns the present or the future, the statement may take a mixed form:

If he had worked hard, he would be rich today.  (Though we're talking about hard work that wasn't done in the past, the possible result of it - i.e. his chance of being rich by now - concerns the present.)

They would come to the party tomorrow if we had invited them at the office yesterday. (Though the result - their attending the party - is about the future, i.e. tomorrow, the time to invite them is already past, i.e. at the office yesterday, and therefore it's no longer possible to invite them.) 
 
If I had studied harder, I wouldn't have to repeat the exam next year. (I didn't study hard enough in the past, so I'll have to repeat the exam in the future.) 

Sometimes it could be the other way round too: the condition may be something that’s valid even at present though the result occurred in the past.

I wouldn't have done it if I were you. (The condition 'if were you' is used in the form of the second conditional because of its timeless quality though the part 'I wouldn't have done it' is about the past.)

You would have understood it if you knew German. (You didn't understand it because you don't know German. The fact that you didn't understand belongs to the past, but even now you don’t know German – i.e. assuming you haven’t learnt the language in the meantime.)

I would have gone on the trip with them if I didn’t have to attend this wedding tomorrow. (I didn’t go on the trip, which has already started, because I have a wedding to attend tomorrow – a planned action that’s yet to come. So I no longer can go on the trip, but my obligation to attend the wedding still stands.)

(Image credit: whatmegsaid)