Mi hermano es un genio.

Because mi is the possessive adjective used before a noun: mi hermano = my brother.

• mi = my
• mí with an accent means me after a preposition, as in para mí
• mío means mine or of mine, and it usually does not go directly before the noun in this structure

Compare:

• mi hermano = my brother
• un hermano mío = a brother of mine

In modern Spanish, a possessive like mi, tu, su, nuestro, etc. normally replaces the article.

So you say:

• mi hermano
• tu casa
• nuestra madre

Not:

• el mi hermano

With family words, Spanish can use the article in other structures, such as el hermano de Ana, but not here with mi.

Because ser is used for identity, classification, and essential characteristics. Here, un genio classifies the brother as a genius.

So:

• Mi hermano es un genio = He is a genius

Using estar here would sound wrong, because estar is usually for states, conditions, or locations, not for saying what someone is in this sense.

Because genio here is a countable noun: a genius.

So Spanish says:

• es un genio

not just:

• es genio

This is different from some nouns after ser, especially professions or nationalities, where Spanish often drops the article:

• Es médico
• Es español

But with an evaluative noun like genio, the article is normally used.

No. In this sentence, genio is a noun, not an adjective.

The structure is:

• Mi hermano = subject
• es = verb
• un genio = noun phrase

If you want an adjective with a similar idea, you could say:

• Mi hermano es genial = My brother is brilliant / great

That is similar, but not exactly the same as es un genio.

They are close, but not identical.

• Es un genio means he is a genius, or someone very clever/talented
• Es genial means he is great, brilliant, or awesome, depending on context

So un genio focuses more on the person as a very clever individual.
Genial is a descriptive adjective and can be broader.

A simple guide is:

mee er-MAH-no es oon HHEH-nyo

A few important points:

• mi sounds like mee, not like English my
• the h in hermano is silent
• the g in genio before e sounds like the strong Spanish throat sound heard in Spain
• genio has two syllables after the first consonant cluster: ge-nio

A more natural Spain pronunciation would be roughly:

Mi ermano es un jénio

with the Spanish j/g sound, not the English j sound.

Yes. Genio can have other meanings in Spanish depending on context, such as:

• temperament or disposition
• in some contexts, genie or magical spirit

But in Mi hermano es un genio, it clearly means genius or very clever person.

No. Spanish often omits subject pronouns when they are not needed.

Here, the subject is already clearly stated as mi hermano, so adding él would be unnecessary.

• Mi hermano es un genio = normal and natural
• Él es un genio = also possible, but it means He is a genius
• Mi hermano, él es un genio = possible only with special emphasis, but not the neutral version

Yes, but Mi hermano es un genio is the normal, neutral order.

You might also hear:

• Es un genio, mi hermano

This sounds more emphatic or expressive, like My brother is a genius, he really is.

For everyday learning, the standard order is the best one to use first:

• Mi hermano es un genio