People sometimes get confused between lay and lie and say something like, “I need to lay down.”
You can’t lay down; you can only lie down. Lie (in this sense) means to assume, or to be in, a horizontal or resting position.
Lay means to put or set [something] down. You can lay a baby in a crib. You can lay a carpet. You can lay foundations. A hen can lay an egg. You can lay down the law. You can lay all sorts of things. But you can’t just lay, or lay down. The verb is transitive. That means the verb must have an object – in other words you can only do it to something or someone.
Lay is also the past tense of lie as in “he lay in bed till ten o’clock”. Perhaps that’s why the words get confused.
You can read about the difference between lay and lie and a whole lot more, in the English Digital Content style guide.
