Je couche le bébé dans son lit

 In French, possessive adjectives agree in gender and number with the noun they modify. In the sentence "Je couche le bébé dans son lit," "son" is used because it agrees with the gender of the noun "lit" (bed), which is masculine singular.

The possessive adjectives in French are as follows:

Masculine singular: "son" (his)

Feminine singular: "sa" (her)

Masculine plural: "ses" (his/their)

Feminine plural: "ses" (her/their)

So, in this case, "son" is used because "lit" is masculine singular. If the sentence were referring to a feminine noun, you would use "sa" instead. For example, "Je couche le bébé dans sa chambre" (I put the baby down in her room), where "chambre" (room) is feminine singular.