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.

 To make a condition less likely you can use phrases (for example, by any chance) or the structureThe way in which the parts of a sentence, clause or expression are arranged. E.g.: make somebody + infinitive in "The teacher made me rewrite the composition."s below.

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)