The transitive verb lay, and lay, the past tense of the neuter verb lie, are often confounded, though quite different in meaning.
The neuter verb to lie, meaning to lie down or rest, cannot take the objective after it except with a preposition. We can say “He lies on the ground,” but we cannot say “He liesthe ground,” since the verb is neuter and intransitive and, as such, cannot have a direct object. With lay it is different. Lay is a transitive verb, therefore it takes a direct object after it; as “I lay a wager,” “I laid the carpet,” etc.
Of a carpet or any inanimate subject we should say, “It lies on the floor,” “A knife lies on the table,” not lays. But of a person we say—”He lays the knife on the table,” not “He lies——.” Lay being the past tense of the neuter to lie (down) we should say, “He lay on the bed,” and lain being its past participle we must also say “He has lain on the bed.”
We can say “I lay myself down.” “He laid himself down” and such expressions.
It is imperative to remember in using these verbs that to lay means to do something, and to lie means to be in a state of rest.