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In grammar, the conditional perfect is a verb form that describes something that might have happened in the past. It is the perfect or completed tense of the conditional mood. The conditional perfect only occurs in independent clauses and has two common uses. One refers to something that might have happened, but did not happen. The other refers to something that might have happened, but the person speaking does not know whether it actually happened or not. The English conditional perfect is formed by combining the conditional "would have" with the past participle of a verb, as in "would have danced."
An example of something that might have happened, but did not is as follows: "If Hugo had known that Katie did not want the last cupcake, then he would have eaten it." In this sentence, "would have eaten" is the conditional perfect. Like many other sentences that use the conditional perfect, this sentence contains two clauses, both of which refer to things that did not actually happen. In this instance, Hugo did not know that Katie did not want the last cupcake, and so he did not eat it.
In more technical terms, the conditional perfect is often found in the apodosis of a conditional, or "if-then," sentence. The clause beginning in "then" is the apodosis and is an independent clause, meaning that it can function as a complete sentence on its own: "Then he would have eaten it." The clause beginning in "if" is known as the protasis and is a dependent clause. For example, "If Hugo had known that Katie didn't want the last cupcake" is not a complete sentence. Although both the apodosis and the protasis refer in this sentence refer to things that did not happen, their grammatical constriction is different. The apodosis is in the conditional perfect form, while the protasis is in the subjunctive mood.
The second use of the conditional perfect has only one clause and is less grammatically complex. An example might be: "Alex probably would have finished his dinner by six thirty." In this case, the speaker assumes that the action — Alex finishing dinner — was completed at some point in the past. There is still uncertainty, however, because the speaker does not have concrete evidence about the state of Alex's dinner.
Construction of the conditional perfect is similar in many other Romance languages. In Spanish, for instance, it is the conditional of hablar — "would have" — plus the past participle. Unlike English, however, the Spanish conditional changes its form depending on the grammatical person and number. Yo habría, meaning "I would have," takes a different form than tú habrías, meaning "you would have."