Minha irmã guarda a moto na garagem do prédio.

Because minha has to agree with the noun irmã, which is feminine.

• meu = my, used with masculine singular nouns
• minha = my, used with feminine singular nouns

So:

• meu irmão = my brother
• minha irmã = my sister

The possessive agrees with the thing owned, not with the speaker.

Here, guarda means keeps, stores, or parks/keeps put away, depending on context.

In this sentence, Minha irmã guarda a moto na garagem do prédio, it suggests that the sister keeps or parks the motorcycle in the building’s garage.

This is from the verb guardar, which often means:

• to keep
• to store
• to put away
• to save
• to guard/protect

So guarda is the third-person singular present tense form:

• eu guardo = I keep
• você/ele/ela guarda = you/he/she keeps

Because a moto means the motorcycle.

In Portuguese, articles are used very often, more often than in English. Here:

• a = the, feminine singular
• moto = motorcycle / motorbike

Since moto is feminine, it takes a:

• o carro = the car
• a moto = the motorcycle

So guarda a moto literally means keeps the motorcycle.

Moto is feminine because it is short for motocicleta, which is a feminine noun.

So even though moto is a shortened form, it keeps the original gender:

• a motocicleta
• a moto

This is something you often just have to learn with the noun.

Na is a contraction of em + a.

• em = in / on / at
• a = the, feminine singular
• em + a = na

So:

• na garagem = in the garage

This kind of contraction is very common in Portuguese:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

So na garagem literally comes from in the garage.

Do is a contraction of de + o.

• de can mean of, from, or indicate possession/relationship
• o = the, masculine singular
• de + o = do

So:

• a garagem do prédio = the garage of the building / the building’s garage

This is a very common structure in Portuguese for possession or connection between nouns.

Other related forms:

• da = de + a
• dos = de + os
• das = de + as

Portuguese usually expresses this idea with de rather than an apostrophe-s structure.

So English:

• the building’s garage

becomes Portuguese:

• a garagem do prédio
• literally: the garage of the building

This is one of the most common ways Portuguese shows possession between nouns.

Prédio usually refers to a building, especially an apartment building, office building, or multi-story urban building.

In this sentence, garagem do prédio strongly suggests the garage belonging to the apartment building or building where she lives.

A native English speaker might wonder whether prédio means specifically apartment building. It does not always have to, but in everyday Brazilian Portuguese it often refers to that kind of building.

Because Portuguese often leaves out the subject pronoun when it is clear from the verb or context.

Here, Minha irmã is already the subject, so there is no need to add ela.

• Minha irmã guarda a moto... = My sister keeps the motorcycle...
• Ela guarda a moto... = She keeps the motorcycle...

Since the noun Minha irmã is already there, adding ela would be unnecessary.

The basic word order is:

subject + verb + object + place

Breaking it down:

• Minha irmã = subject
• guarda = verb
• a moto = direct object
• na garagem do prédio = location

So the sentence follows a very familiar order for English speakers:

My sister / keeps / the motorcycle / in the building’s garage.

That makes this sentence structurally quite straightforward.

Not usually. Although guardar is related to the idea of guarding, in everyday Portuguese it very often means:

• to keep
• to store
• to put away

So in many contexts, translating it as guard would sound unnatural in English.

Examples:

• Guardei os documentos na gaveta. = I kept/put the documents away in the drawer.
• Ela guarda dinheiro. = She saves money.
• Ele guarda o carro na garagem. = He keeps/parks the car in the garage.

So in this sentence, think of guarda as keeps or parks/stores, not usually guards.

In careful pronunciation, it would sound roughly like:

minha irmã guarda a moto na garagem do prédio

A few useful pronunciation notes:

• minha often sounds close to MEE-nya or even more smoothly compressed in fast speech
• irmã has a nasal ending; the ã is not a normal a
• guarda has a hard g sound, like in guard
• moto is usually MOH-too
• garagem in Brazilian Portuguese often ends with a nasal sound, roughly gara-ZHEM
• prédio has stress on pré

A very rough English-friendly approximation might be:

MEE-nya eer-MAHN GWAR-da a MOH-tu na gara-ZHEN do PREH-dyoo

But it is better to treat that only as a rough guide, not an exact pronunciation system.

Yes, you could, but it changes the feel slightly.

• Minha irmã guarda a moto... = My sister keeps the motorcycle...
• Minha irmã guarda sua moto... = My sister keeps her motorcycle...

Using a moto just refers to the motorcycle, and the ownership may be understood from context. Using sua moto makes the ownership explicit.

However, seu/sua in Portuguese can sometimes be ambiguous, because it can mean:

• your
• his
• her
• its

depending on context.

So Brazilian Portuguese often prefers clearer alternatives such as:

• a moto dela = her motorcycle
• a sua moto = your motorcycle, in some contexts

So yes, sua moto is possible, but a moto is very natural if the meaning is already clear.