Mi abuelo es cubano.

Because mi is the short possessive adjective used before a noun: mi abuelo = my grandfather.

Mío is the long possessive form, and it is usually used after a noun or by itself:

• Este abuelo es mío. = This grandfather is mine.

So before abuelo, you need mi, not mío.

We use es because the sentence is identifying a person’s origin/nationality, which is treated as an essential characteristic.

• Mi abuelo es cubano. = My grandfather is Cuban.

In Spanish, ser is used for things like identity, nationality, profession, and permanent characteristics.

Estar is used more for location, condition, or temporary states:

• Mi abuelo está en Cuba. = My grandfather is in Cuba.

So es cubano is correct, not está cubano.

In Spanish, nationality words are usually not capitalized unless they begin a sentence.

So:

• Mi abuelo es cubano.
• Ella es mexicana.
• Somos argentinos.

This is different from English, where Cuban, Mexican, and Argentine/Argentinian are capitalized.

Because abuelo is masculine singular, the nationality adjective must agree with it in gender and number.

• abuelo → masculine singular
• so: cubano

Compare:

• Mi abuelo es cubano.
• Mi abuela es cubana.
• Mis abuelos son cubanos.
• Mis abuelas son cubanas.

This is a very common pattern in Spanish: adjectives often change to match the noun.

Spanish often omits subject pronouns because the verb form already tells you who the subject is.

Here, es can mean he/she/it is, but the noun mi abuelo already tells us the subject clearly. So there is no need to add él.

• Mi abuelo es cubano. = natural
• Él es cubano. = He is Cuban
• Mi abuelo, él es cubano. = usually unnecessary unless you are emphasizing or contrasting

Spanish prefers to avoid pronouns when they are not needed.

Yes. That is also correct, but it is slightly different in structure.

• Mi abuelo es cubano. = My grandfather is Cuban.
• Mi abuelo es de Cuba. = My grandfather is from Cuba.

Both communicate origin, and in many contexts they are very close in meaning.
The first uses a nationality adjective (cubano).
The second uses de + place of origin (de Cuba).

Because in Spanish, after ser, a nationality adjective is used directly, without an article.

• Es cubano.
• Es peruana.
• Son chilenos.

You do not say:

• es un cubano in this sentence if you simply mean nationality

Un cubano means a Cuban person, as a noun:

• Mi abuelo es un cubano muy orgulloso. = My grandfather is a very proud Cuban man/person.

So in your sentence, cubano is functioning as an adjective, not a noun.

Yes, but Mi abuelo es cubano is the most neutral and natural order.

You might also hear:

• Es cubano mi abuelo.

That version is more marked and may sound poetic, emphatic, or contrastive depending on context. For normal conversation, stick with:

• Mi abuelo es cubano.

Spanish word order is somewhat flexible, but the standard pattern is: subject + verb + complement

You change the possessive, the noun, the verb, and the adjective as needed:

• Mi abuelo es cubano. = My grandfather is Cuban.
• Mis abuelos son cubanos. = My grandparents / grandfathers are Cuban.

Breakdown:

• mi → mis
• abuelo → abuelos
• es → son
• cubano → cubanos

Everything has to agree correctly.

A simple English-friendly guide is:

• abuelo ≈ ah-BWEH-loh
• cubano ≈ koo-BAH-noh
• mi ≈ mee
• es ≈ ess

A few helpful notes:

• In abuelo, the ue sounds like we in many pronunciations: a-bwe-lo
• In cubano, the c before u sounds like k
• Spanish vowels are usually clear and pure, not heavily reduced like in English

The natural stress is:

• abuelo
• cubano

By itself, abuelo usually means grandfather, but in real life it can also be used affectionately like grandpa, depending on tone and context.

If you want a more affectionate or familiar form, you may also hear:

• abuelito

That often sounds like grandpa or little grandpa in an affectionate way.

So:

• Mi abuelo es cubano. = neutral, standard
• Mi abuelito es cubano. = more affectionate

Because mi does not change for gender in the singular.

• mi abuelo = my grandfather
• mi abuela = my grandmother

It only changes for number:

• mi = singular
• mis = plural

So:

• mi abuelo
• mi abuela
• mis abuelos
• mis abuelas

This is different from some other Spanish words that do change for gender.