Minha cunhada prefere ir de ônibus, porque o trânsito fica mais lento à tarde.

Because cunhada is a feminine noun in Portuguese, so the possessive adjective has to agree with it.

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

Examples:

• meu irmão = my brother
• minha irmã = my sister
• meu cunhado = my brother-in-law
• minha cunhada = my sister-in-law

So minha cunhada is correct because cunhada is feminine.

Cunhada means sister-in-law.

It is the feminine form of:

• cunhado = brother-in-law
• cunhada = sister-in-law

This word can refer to:

• your spouse’s sister, or
• your sibling’s wife

In everyday Brazilian Portuguese, it is a very common family word.

This is a very common structure in Portuguese.

• prefere = prefers
• ir = to go

After the verb preferir in this kind of sentence, Portuguese often uses an infinitive verb to say what someone prefers to do.

So:

• prefere ir = prefers to go

Other examples:

• Ela prefere ficar em casa. = She prefers to stay home.
• Eu prefiro estudar à noite. = I prefer to study at night.

This works similarly to English prefer to go, prefer to stay, prefer to study.

Because prefere is the conjugated form, while preferir is the infinitive.

Here the subject is minha cunhada, which is she. So the verb must be in the third person singular:

• eu prefiro = I prefer
• você/ele/ela prefere = you/he/she prefers
• nós preferimos = we prefer
• eles/elas preferem = they prefer

Since minha cunhada = she, the correct form is prefere.

In Portuguese, de is commonly used to say the means of transportation.

So:

• ir de ônibus = to go by bus
• ir de carro = to go by car
• ir de trem = to go by train
• ir de avião = to go by plane

This is one of those patterns you mostly just learn as a chunk:

• de + transport

It does not literally work like English word-for-word. You should think of ir de ônibus as the normal Portuguese way to say go by bus.

Because when Portuguese talks about a means of transportation in a general way, it normally uses just:

• de ônibus
• de carro
• de metrô

No article is needed here.

Compare:

• Ela vai de ônibus. = She goes by bus.
• Ela vai no ônibus. = She goes on the bus / in the bus

These mean different things:

• de ônibus = by bus, talking about the method of transport
• no ônibus = on the bus, referring to the actual vehicle

So in your sentence, de ônibus is correct because it expresses how she goes.

Here porque means because, so it is written as one word.

This is one of the most common uses:

• porque = because

Examples:

• Fiquei em casa porque choveu. = I stayed home because it rained.
• Ela foi cedo porque estava cansada. = She left early because she was tired.

A quick comparison:

• porque = because
• por que = why / for what reason
• por quê = why? at the end of a sentence
• o porquê = the reason

In your sentence, the second clause gives a reason, so porque is the right form.

O trânsito means traffic.

Portuguese usually treats this idea as a singular mass noun, just like English often does.

So:

• o trânsito está ruim = traffic is bad
• o trânsito fica lento = traffic gets slow
• há muito trânsito = there is a lot of traffic

Even though traffic involves many cars, the noun itself is singular.

Also:

• trânsito can refer to traffic conditions in general
• in other contexts, it can also relate to transit or circulation, but traffic is the natural meaning here

Yes, ficar can mean to stay, but it has several common meanings in Portuguese. One very common one is:

• ficar + adjective = to become / to get

So in this sentence:

• o trânsito fica mais lento = traffic becomes slower / traffic gets slower

This is very natural Portuguese.

Examples:

• Ela ficou triste. = She got sad.
• A rua fica perigosa à noite. = The street becomes dangerous at night.
• O café fica frio rápido. = The coffee gets cold quickly.

So here fica is not really stays. It is closer to gets or becomes.

Yes. Mais lento means slower or literally more slow.

Portuguese forms most comparatives with:

• mais + adjective = more ...
• menos + adjective = less ...

So:

• mais lento = slower
• mais rápido = faster
• mais caro = more expensive

Examples:

• O trânsito fica mais lento à tarde. = Traffic gets slower in the afternoon.
• Esse carro é mais rápido. = This car is faster.

Notice that Portuguese often uses mais + adjective where English may use -er:

• mais lento = slower
• mais alto = taller
• mais fácil = easier

Because lento describes o trânsito, and trânsito is a masculine noun.

Adjectives in Portuguese usually agree with the noun they describe.

• o trânsito lento = the slow traffic
• a rua lenta would sound odd semantically, but grammatically lenta would match a feminine noun

So in your sentence:

• o trânsito = masculine singular
• therefore lento = masculine singular

If the noun were feminine, the adjective would change:

• a viagem ficou mais lenta = the trip got slower

À tarde means in the afternoon.

The à is a contraction of:

• a + a = à

This is called crase in Portuguese.

In time expressions, à often appears in fixed phrases like:

• à tarde = in the afternoon
• à noite = at night / in the evening
• às três horas = at three o’clock

Important comparison:

• a tarde without the accent could mean the afternoon as a noun
• à tarde means in the afternoon

So the accent matters.

Sometimes yes, but not with exactly the same feel.

• à tarde is the most common general expression for in the afternoon
• na tarde de segunda-feira = on Monday afternoon
• na tarde daquele dia = in the afternoon of that day

So:

• O trânsito fica mais lento à tarde. = general statement, very natural
• O trânsito ficou lento na tarde de ontem. = referring to a specific afternoon

In your sentence, à tarde is the best choice because it talks about afternoons in general.

Yes, the reason can come first, but the original order is the most straightforward.

Original:

• Minha cunhada prefere ir de ônibus, porque o trânsito fica mais lento à tarde.

You can also say:

• Porque o trânsito fica mais lento à tarde, minha cunhada prefere ir de ônibus.

Both are grammatical, but the first one sounds more natural in everyday speech because it starts with the main idea and then gives the reason.

This is very common in Portuguese:

• statement first
• reason after porque

In Brazilian Portuguese, ônibus is pronounced approximately like OH-nee-boos, with stress on the first syllable.

The accent mark in ô shows:

• the stressed syllable is there, and
• the vowel is a closed o sound

So:

• Ô-ni-bus

Also, ônibus is an interesting word because its singular and plural forms are the same:

• um ônibus = a bus
• dois ônibus = two buses

The article tells you whether it is singular or plural.

Yes, it sounds natural.

It uses several very common everyday patterns:

• family vocabulary: minha cunhada
• preference verb: prefere
• transportation expression: ir de ônibus
• reason clause with porque
• common traffic vocabulary: o trânsito
• time expression: à tarde

A Brazilian speaker would understand it immediately, and it sounds like normal, real-life Portuguese.