Buenas noches, abuela.

Because noches is a feminine plural noun.

• noche = night, evening
• noches = nights
• bueno/buena/buenos/buenas changes to match the noun

So:

• buenas noches = feminine plural
• buenos días = masculine plural
• buenas tardes = feminine plural

This is basic adjective agreement in Spanish.

This is just how the fixed expression works in Spanish. Spanish uses the plural:

• buenos días
• buenas tardes
• buenas noches

Even though English often uses the singular, Spanish normally uses the plural in these greetings. You should learn buenas noches as a set phrase.

It can mean both, depending on the situation.

In Latin American Spanish, buenas noches is used:

• as a greeting in the evening or at night = good evening
• as a farewell at night or before bed = good night

So context tells you which meaning is intended.

The comma is used because abuela is a direct address word, also called the vocative.

You are speaking to your grandmother, not just mentioning her.

• Buenas noches, abuela. = Good evening/night, Grandma.

The comma helps show that abuela is the person being addressed.

When you are talking to a family member directly, Spanish often uses just the family word by itself:

• Mamá
• Papá
• Abuela
• Tío

So Buenas noches, abuela sounds natural.

If you say mi abuela, that usually sounds more like you are talking about her, not directly addressing her.

• Mi abuela está aquí. = My grandmother is here.

Normally, no. In standard Spanish, family words like abuela, mamá, or tío are usually not capitalized unless they begin the sentence.

So:

• Buenas noches, abuela. ✔
• Buenas noches, Abuela. usually not standard in normal writing

Some people may capitalize family words in personal messages or cards for stylistic reasons, but the normal rule is lowercase.

A simple approximate pronunciation is:

BWEH-nas NO-chehs, ah-BWEH-lah

A few helpful notes:

• bu in buenas sounds like bweh
• ch in noches sounds like English ch
• ll in abuela in most of Latin America sounds like y or a soft j sound depending on the region, but in abuela there is no ll—just a regular l
• Stress:
  • BUE-nas
  • NO-ches
  • a-BUE-la

The greeting itself is neutral and works in both formal and informal situations.

• Buenas noches can be said to almost anyone
• Adding abuela makes it personal because you are speaking to your grandmother

So the sentence is natural and respectful, but not especially formal.

Yes.

• Hola, abuela = simple Hi, Grandma
• Buenas noches, abuela = more specific to the time of day
• Hola, buenas noches, abuela = possible, though a little more expanded

In real conversation, people often use either Hola or Buenas noches by itself, depending on how warm or complete they want the greeting to sound.

Yes, if that fits your relationship.

Abuelita is a diminutive form of abuela. It often sounds:

• more affectionate
• more tender
• sometimes more childlike or intimate

So:

• Buenas noches, abuela = normal, natural
• Buenas noches, abuelita = sweeter / more affectionate

Whether it sounds right depends on the family and region, but it is very common in Latin America.