In Spanish, the phrase ‘lay down’ can be translated as ‘acostarse’ or ‘tumbarse’, depending on the context.
Acostarse is used when referring to lying down to rest or sleep. For example, ‘Me voy a acostar’ means ‘I am going to lay down (to sleep).’
Tumbarse is used when referring to lying down in a more general sense, such as lying on the ground or on a surface. For example, ‘Se tumbó en la hierba’ means ‘He/She lay down on the grass.’
Both verbs are reflexive, meaning they require a reflexive pronoun (me, te, se, nos, os, se) to indicate that the action is being performed on oneself.