The second conditional in English is used to talk about imaginary scenarios that the speaker considers unlikely to happen or to be true.
She wouldn't be happy if she found out about it. (But I don't quite think she'll find out.)
If I had a million dollars, I'd buy a Porsche. (But I don't have that much money and it's not likely that I'll get it anytime soon.)
If I were you, I wouldn't do that. (But there's no way I can be you.)
In such sentences, the verb in the 'if-clause' doesn't really mean some past action but something improbable, if not downright impossible. In fact, though he verb form used seems to be in the past tense, it's really in the so-called subjunctive mood (past subjunctive); the past subjunctive is used not only for remote possibilities like in the second conditional, but also to express a wish, emotion etc.
I wish I had a car. (But I don't have one at the moment.)
If only they were here! (But they're not.)
Unlike in some other European languages, the past subjunctive in modern English resembles the past tense in the case of most verbs. The rare exception is the verb 'be', where 'were' is used - especially in formal usage - even with a singular subject (as in 'If I were you'). This lack of difference in most verbs spares the second-language learner the trouble of memorizing one more verb form, but leads to some difficulty in separating it from real past action.
On the other hand, as standard grammar books present the second conditional as if any 'if-clause' with a past-tense verb should fall in that category, some students have trouble realizing that 'if' can occur with real past action too.
If he had some free time, he played video games. (Whenever he had some free time, he played video games.)
If she found out about it, why didn't she talk about it?. (Assuming that she found out about it...)
If he was at school yesterday, he must have seen the accident. (Assuming that he was at school yesterday...)
(Image credit: M. Pratter)