What is the past participle of lain?

The past participle of "lain" is lain. It's the past participle of the verb "to lie" when it means "to rest in a horizontal position".

Here's a breakdown:

* Lie (present tense): I lie down on the couch.

* Lay (past tense): I laid down on the couch yesterday.

* Lain (past participle): I have lain here for hours.

It's important to remember that "lay" is a different verb, meaning "to place something down." Its past participle is laid.