Conditional perfect

The conditional perfect is a grammatical construction that combines the conditional mood with perfect aspect. A typical example is the English "would have written". The conditional perfect is used to refer to a hypothetical, usually counterfactual, event or circumstance placed in the past, contingent on some other circumstance (again normally counterfactual, and also usually placed in the past). Like the present conditional (a form like "would write"), the conditional perfect typically appears in the "apodosis" (the main clause, expressing the consequent) in a conditional sentence.
English.
In English, the conditional perfect is formed using "would have" together with the past participle of the main verb. The auxiliary "would" marks the conditional mood (it is occasionally replaced by "should" in the first person; see "shall" and "will"), while the auxiliary "have" (used in combination with the past participle) marks the perfect aspect (prior occurrence of the event in question). The conditional perfect is used chiefly in the main clause (apodosis) of "third conditional" (or sometimes "mixed conditional") sentences, as described under English conditional sentences. Examples:
It is also possible for the auxiliary "would" to be replaced by the modals "should", "could" or "might" to express appropriate modality in addition to conditionality.
Sometimes, in (chiefly American English) informal speech, the "would have" construction appears in the "if"-clause as well ("If we would have run faster, we would have arrived earlier"), but this is considered incorrect in formal speech and writing (see ).
English also has a conditional perfect progressive ("would have been writing"). For more details on the usage of this and of the ordinary conditional perfect, see the relevant sections of the article Uses of English verb forms.
Other languages.
French expresses past counterfactual conditional sentences in exactly the same way as English does: the "if" clause uses the "had" + past participle (pluperfect) form, while the "then" clause uses the "would have" + past participle form, where the equivalent of "would have" is the conditional of the auxiliary ("avoir" or "être") used in all perfect constructions for the verb in question. Example:
Spanish forms the conditional perfect on similar principles, e.g. "yo te habría dicho todo" ("I would have told you everything").
Dutch has a similar tense to the English one, formed with "zou"/"zouden", the past tense of "zullen", the auxiliary of the future tenses, e.g. "ik zou je alles gezegd hebben" ("I would have told you everything"). In Dutch grammar it is called the "perfect past future tense", emphasizing that it also has future-in-past properties.
For certain other languages, see conditional mood.