Since there’s no verb in the phrase, it doesn’t change regardless of who you’re addressing. “Buenas noches” can be used either as a greeting or a farewell, as long as it’s dark out. However, it’s more commonly used as a greeting.
For example, if you were meeting your in-laws for the first time, you might say “feliz noche” to them as you were parting ways. Another polite farewell to use late in the evening is “que tengan buena noche” (kay tehn-GEHN boo-EHN-ah noh-CHAY), which means “have a good night. "
If you’re saying farewell to a group of people, say (vosotros) “descansad” or (ustedes) “descansen,” depending on your level of familiarity with the people and the customs of the country. This is a more casual farewell, typically used when you’re on more familiar terms with the people you’re addressing.
Use this conjugation when you’re speaking to children, or to friends and family members with whom you’re on familiar, casual terms.
This would also be the form to use if you’re talking to someone you don’t know very well, such as a shop clerk or a friend of a friend who you’ve just met. If you’re speaking to a group of people, say “que pasen buenas noches (ustedes). "
Formally, the phrase is “que tenga buenas noches. " The plural is “que tengan buenas noches. " In normal conversation, people typically don’t include the pronoun “usted. "
Tú: “Que duermas bien. " Usted: “Que duerma bien. " Vosotros: “Que durmáis bien. " Ustedes: “Que duerman bien. "
Tú: “¡Duerme bien!” Usted: “¡Duerma bien!” Ustedes: “¡Duerman bien!”
This phrase typically is only used for children – occasionally for younger siblings or significant others. Since the phrase is only used familiarly, you only need the familiar conjugations of the verb tener. Use tengas if you’re addressing one person, and tengáis if you’re addressing several people. You also can shorten the phrase and simply say “dulces sueños,” or “sweet dreams. “[13] X Research source
This phrase uses the verb soñar (“to dream”), which is conjugated irregularly. However, since it’s only used for children, the familiar conjugations are the only ones you need to know: sueñes (singular) and soñéis” (plural). You can also say this in the command form: “Sueña con los angelitos. "
