Now this one is more for me, than you. These four words are a curse to me, because I cannot remember which word I should be using. I’ve been told a million times, I had a friend send me a photocopy of the rule, but still I don’t know.
(Note: The following usages are for Australians. They might change elsewhere in the world, so please — please — do not confuse me by telling me it’s done differently elsewhere. I need to know how it’s done in my own country.)
Laid is the past tense of the verb to lay. You always lay something in some place. The hen laid an egg; She laid the plates on the table.
Lain is the past tense of the verb to lie. You lie or rest somewhere. I have lain on the bed for a rest.
Lay is a verb meaning to put something down. To lay a path in the garden.
Lie is a verb meaning to “lie down”, “to be at rest”. It is dangerous to lie in the sun. It can also mean to tell an untruth. Do not lie to me. Lie can also be a noun. That is a lie; you know it is untrue.
Source: The Foundation Grammar Dictionary by Gordon Winch