The past perfectThe aspect expressing completion. E.g.: He’s finished university. subjunctive has the same form as the past perfect tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses..

had + past participle

It is used in subordinate clauseA clause that cannot be a sentence in itself. It is joined with a main clause. E.g.: I wouldn’t go there if I were you. ("if I were you") When I arrived he was surfing. ("when I arrived")s and expresses a hypotheticalUnreal, imagined. past situation.

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 concept of hypothesis is discussed in datails on the page Modals used in conditional sentences.

ClauseA group of related words containing a subject and a verb.s that start with as if/as though express doubt or uncertainty if they are followed by an unrealA tense that expresses a hypothetical situation. E.g.: I wish you were here. tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses.. Otherwise, they express that the statement is true or might be true.

 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)

The unrealA tense that expresses a hypothetical situation. E.g.: I wish you were here. (or hypotheticalUnreal, imagined.) tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses.s have the form of the past simple or the past perfectThe aspect expressing completion. E.g.: He’s finished university. but they refer to present or past timeA concept which is related to our perception of reality; that part of existence which is measured in seconds, minutes, hours, days, weeks, etc., respectively. They are used in subordinate clauseA clause that cannot be a sentence in itself. It is joined with a main clause. E.g.: I wouldn’t go there if I were you. ("if I were you") When I arrived he was surfing. ("when I arrived")s which hypothesise an imaginary present or an impossible past situation.