* Lie (present tense) - I lie down for a nap.
* Lay (past tense) - I lay down for a nap yesterday.
* Lain (past participle) - I have lain down for a nap every day this week.
* Lying (present participle) - I am lying down for a nap now.
Important Note: The verb "lie" can also mean "to tell an untruth," and has different principle parts. However, this question is specifically asking about the verb "lie" meaning "to rest or recline."