The most common verb forms of lay are:
lay (present), laid (past), laid (past participle) and laying (present participle).
While the most common verb forms of lie are:
lie (present), lay (past), lain (past participle) and lying (present participle).
Furthermore, lay means to place something or to put something on something, while lie means to recline. In other words, "lay" means the subject is acting on something or someone else, and so must take a direct object. But, "lie" means the actor is doing something to herself, and so is a complete verb.
So, as someone who usually writes in past tense, and often has her characters recline, I need to remember lay! How about you? Do you lie? lay? get laid? :)
Why do I have so much trouble remembering this? It's a mystery.
Good luck with your own grammatical conundrums. :)