Minha irmã ficou tonta no ônibus e precisou de água.

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

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

Examples:

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

So the possessive matches the noun being possessed, not the speaker.

Irmã means sister, while irmão means brother.

They are the feminine and masculine forms of the same family word:

• irmão = brother
• irmã = sister

So minha irmã specifically tells you the person is female.

Here, ficar means to become or to end up in a certain state.

So:

• ficou tonta = became dizzy / got dizzy

This is very common in Portuguese:

• ficar feliz = to become happy
• ficar triste = to become sad
• ficar doente = to get sick

In this sentence, the idea is not just that she was dizzy, but that she became dizzy at that moment.

Because adjectives in Portuguese usually agree with the noun they describe.

Since irmã is feminine singular, the adjective also has to be feminine singular:

• tonto = dizzy, masculine
• tonta = dizzy, feminine

Examples:

• meu irmão ficou tonto
• minha irmã ficou tonta

Ficou is the preterite form of ficar for ele/ela/você.

• infinitive: ficar
• preterite: eu fiquei, você/ele/ela ficou, nós ficamos...

The preterite is used for a completed event in the past. In this sentence, it refers to a specific moment: she got dizzy on the bus.

So ficou tonta sounds like:

• she got dizzy
• she became dizzy

If you said ficava tonta, that would suggest something habitual or repeated in the past.

You can say estava tonta, but it changes the meaning a little.

• ficou tonta = she got dizzy / became dizzy
• estava tonta = she was dizzy

So:

• ficou focuses on the change of state
• estava focuses on the state itself

In your sentence, ficou tonta is natural because it describes something that happened during the bus ride.

No is a contraction of:

• em
  • o = no

So:

• no ônibus literally = in/on the bus

Portuguese often uses em with means of transportation:

• no ônibus = on the bus
• no carro = in the car
• no trem = on the train

This contraction is very common:

• em + a = na
• em + os = nos
• em + as = nas

Examples:

• na escola
• nos Estados Unidos
• nas férias

Because prepositions do not match word-for-word between languages.

English says:

• on the bus

Portuguese says:

• no ônibus = literally something like in the bus, but this is just the normal Portuguese way to express it

So it is best to learn no ônibus as the natural expression, rather than trying to translate the preposition directly.

The accent in ônibus shows the stressed syllable and helps mark the correct pronunciation.

• ônibus is stressed on the first syllable: Ô-ni-bus

Also, ônibus is one of those nouns whose singular and plural often look the same:

• o ônibus = the bus
• os ônibus = the buses

The article usually tells you whether it is singular or plural.

Because the verb precisar, when it means to need, normally uses the preposition de before a noun.

So:

• precisar de água = to need water
• precisar de ajuda = to need help
• precisar de dinheiro = to need money

That is why the sentence says:

• precisou de água = needed water

This is a very important pattern to remember:

• precisar de + noun

De água is natural here because it means she needed water in a general sense, like some water.

• precisou de água = she needed water / some water

If you said precisou da água, that would usually mean she needed the water, referring to a specific water already known in the conversation.

Compare:

• precisou de água = needed water
• precisou da água que estava na mochila = needed the water that was in the backpack

So the version in your sentence is the normal one.

Yes. In Brazilian Portuguese, it is often possible to use the definite article before possessives:

• minha irmã
• a minha irmã

Both are possible. The version without the article is completely natural, especially in a simple sentence like this.

Usage can vary by region, style, and level of formality, but both are common in Brazil.

Because Portuguese often does not need an explicit subject pronoun when the subject is already clear.

Here, the subject is already stated:

• Minha irmã ficou tonta...

After that, there is no need to add ela.

Portuguese often omits subject pronouns because the verb form and context usually make the subject clear. Also, repeating the pronoun would often sound unnecessary.

Precisou is also in the preterite, which matches the completed past event in the sentence.

• ficou tonta = got dizzy
• precisou de água = needed water

Both actions are presented as specific things that happened during that bus ride.

If you used precisava, it would sound more like an ongoing or background need:

• ela estava tonta e precisava de água = she was dizzy and needed water

That is also possible, but it gives a slightly different feel. The original sentence is more event-based and concise.

E simply means and.

It links the two actions:

• ficou tonta
• precisou de água

So the structure is:

• My sister got dizzy on the bus and needed water.

This is a very basic and common connector in Portuguese.