Lie is an intransitive verb (a noun can’t be directly next to the verb) meaning to recline or rest on a surface. Its 3 forms are lie, lay (past), lain (past participle). It is often used with ‘down’ or ‘down on’.
I don’t feel well; I need to lie down!
Lay is a transitive verb (a noun comes right after the verb) meaning to put or place somewhere. Its forms are lay and laid (past and past participle). It is often used with ‘on’.
He laid the book on the table.
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