Am I the only one who can’t ever get this right?
Now here’s one of the best explanations I have found and I’m posting it here so that I can find it again. Courtesy of Dictionary.com:
Lay 1 and lie 2 are often confused. Lay is most commonly a transitive verb and takes an object. Its forms are regular. If “place” or “put” can be substituted in a sentence, a form of lay is called for: Lay the folders on the desk. The mason is laying brick. She laid the baby in the crib. Lay also has many intransitive senses, among them “to lay eggs” ( The hens have stopped laying ), and it forms many phrasal verbs, such as lay off “to dismiss (from employment)” or “to stop annoying or teasing” and lay over “to make a stop.”
Lie, with the overall senses “to be in a horizontal position, recline” and “to rest, remain, be situated, etc.,” is intransitive and takes no object. Its forms are irregular; its past tense form is identical with the present tense or infinitive form of lay : Lie down, children. Abandoned cars were lying along the road. The dog lay in the shade and watched the kittens play. The folders have lain on the desk since yesterday.
In all but the most careful, formal speech, forms of lay are commonly heard in senses normally associated with lie. In edited written English such uses of lay are rare and are usually considered nonstandard: Lay down, children. The dog laid in the shade. Abandoned cars were laying along the road. The folders have laid on the desk since yesterday.