Should and were to can also be used in the if-clauseA group of related words containing a subject and a verb. of third conditional sentences. To express that the sentence refers to past, however, a perfectThe aspect expressing completion. E.g.: He’s finished university. infinitive is used after them. Note that this usage (like the situation it expresses) is not very common.

subject 1 + would rather/sooner + clause (subject 2 + past perfect)

I’d rather we had never met. (= If only we had never met.)
What would you rather I had written? (= What do you wish I had written?)

subject 1 + would rather/sooner + clause (subject 2 + unreal tense)

In second conditional, the past subjunctive form of be to is used to express that a condition is less likely.

In this type of mixed conditional the if-clauseA group of related words containing a subject and a verb. is in the second, while the main clauseA clause that could be a sentence in itself. E.g.: I wouldn’t go there if I were you. ("I wouldn't go there") When I arrived he was surfing. ("he was surfing") is in the third conditional.

If Mr Douglas had a moustache, Ms Zeta Jones would probably not have married him.
If I were smarter, I would have been able to answer.

Compare:

In this type of mixed conditional the if-clauseA group of related words containing a subject and a verb. is in the third, while the main clauseA clause that could be a sentence in itself. E.g.: I wouldn’t go there if I were you. ("I wouldn't go there") When I arrived he was surfing. ("he was surfing") is in the second conditional. 

If you had smoked less, you wouldn't be sick now.
If Mr Douglas had had a moustache, Zeta Jones wouldn't be his wife now.
If we had followed the sign, we wouldn’t be lost now.

The third conditional describes an unrealA tense that expresses a hypothetical situation. E.g.: I wish you were here. situation with reference to the past. We hypothesise a past situation which did not happen and cannot happen any more. It is also called the impossible condition.

if-clause with past perfect simple or continuous
main clause with would + perfect infinitive (simple or continuous)

The second conditional usually describes an unrealA tense that expresses a hypothetical situation. E.g.: I wish you were here. situation with reference to the present. We imagine a hypotheticalUnreal, imagined. situation, which is in contrast with the real situation in the present.

if-clause with past simple or continuous
main clause with would + infinitive (simple or continuous)