What is the past tense of lain?

The past tense of "lain" is already in the past tense. It's the past participle of "lie" (to recline).

The past tense of the verb "lie" (to recline) is lay.

Here's how to use them:

* Lay: I lay down for a nap. (past tense of "lie")

* Lain: I have lain in bed all day. (past participle of "lie")