What is the past participle of foolish?

Answer 1

The word "foolish" is an adjective. It describes a noun. ("He asked a foolish question.") Only a verb can have a past participle, and since "foolish" is not a verb, it does not have any past participle. On the other hand, "fool" can be used as a noun ("Don't act like a fool!") or it can be used as a verb-- to fool someone. Using it as a verb gives it a past tense (he fooled his friends with magic tricks); and with the participle, it would be used like this: "He has fooled many people over the years."

Answer 2

The past participle of "foolish" is "foolish" itself.