Lying as in "telling a lie":
* Mentir: This is the most common and general term for lying.
* Dire un mensonge: This literally means "to say a lie," and is a more formal way of saying "to lie."
* Tromper: This means "to deceive" and can be used when someone is deliberately misleading someone else.
Lying as in "being in a recumbent position":
* Être couché: This means "to be lying down."
* S'allonger: This means "to lie down" and is used when someone is actively lying down.
Example sentences:
* Il ment à sa femme: He's lying to his wife.
* Elle a dit un mensonge: She told a lie.
* Il m'a trompé sur le prix: He deceived me about the price.
* Je suis couché sur le canapé: I'm lying on the couch.
* S'allongez sur le dos: Lie on your back.
The best translation will depend on the specific context of the sentence.