Com a chuva, o trânsito fica mais lento.

In Portuguese, com does not only mean with in a literal sense. In sentences like this, com a chuva often expresses a situation or condition: with the rain / when there is rain / because of the rain.

So Com a chuva, o trânsito fica mais lento is a very natural way to say that rain causes traffic to slow down.

Other possible ways to express a similar idea are:

• Quando chove, o trânsito fica mais lento. = When it rains, traffic gets slower.
• Por causa da chuva, o trânsito fica mais lento. = Because of the rain, traffic gets slower.
• Na chuva, o trânsito fica mais lento. = In rainy conditions, traffic gets slower.

Portuguese uses definite articles much more often than English does.

• a chuva = the rain
• o trânsito = the traffic

Even when English would often just say rain or traffic, Portuguese frequently uses the article:

• A chuva atrapalha. = Rain gets in the way.
• O trânsito está ruim. = Traffic is bad.

So com chuva might be possible in some contexts, but com a chuva sounds more complete and natural in this sentence.

Ficar here means to become, to get, or to end up being.

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

If you used é:

• O trânsito é mais lento

that would sound more like a general, fixed description, and it would usually need a comparison or more context. In this sentence, the idea is that rain changes the situation, so fica is the natural choice.

Compare:

• Ele fica nervoso. = He gets nervous.
• Ele é nervoso. = He is a nervous person.

In Portuguese, trânsito is normally treated as a singular mass noun, just like traffic in English.

So:

• O trânsito está lento.
• O trânsito fica pior à noite.

You would not normally make it plural to mean road traffic in general. It refers to the overall flow or condition of vehicles on the roads, not to individual cars.

Both can work, but they are used a little differently.

• lento is an adjective: slow
• devagar is usually an adverb: slowly

In this sentence, trânsito is a noun, so using an adjective is very natural:

• O trânsito fica mais lento. = Traffic gets slower.

You may also hear:

• O trânsito anda mais devagar. = Traffic moves more slowly.

That version focuses more on movement. The original sentence describes the condition of traffic, so mais lento fits very well.

Because it agrees with trânsito, which is masculine singular.

• o trânsito → masculine singular
• therefore lento → masculine singular

If the noun changed, the adjective would change too:

• A viagem fica mais lenta. = The trip gets slower.
• As ruas ficam mais lentas is grammatically possible, but less natural semantically depending on context.

The important idea is adjective agreement: in Portuguese, adjectives often match the noun in gender and number.

The comma separates the introductory phrase from the main clause.

• Com a chuva, = with the rain / when it rains
• o trânsito fica mais lento. = traffic gets slower

This comma is very common when a sentence begins with a short prepositional phrase. It helps readability.

You may sometimes see the sentence without the comma:

• Com a chuva o trânsito fica mais lento.

That is still understandable, but the comma is very natural and polished here.

Yes, you can. It is natural, but the nuance is slightly different.

• Com a chuva = with the rain / because of the rain
• Na chuva = in the rain / under rainy conditions

Both are common and idiomatic.
Com a chuva may sound a bit more like rain as the cause.
Na chuva may sound a bit more like the condition people are experiencing.

In everyday Brazilian Portuguese, both are possible.

Usually trânsito refers to road traffic: cars, buses, motorcycles, and general movement on streets and roads.

Examples:

• O trânsito está parado. = Traffic is stopped.
• Peguei muito trânsito hoje. = I hit a lot of traffic today.

In some contexts, it can relate more broadly to circulation or movement, but for learners, the safest meaning is road traffic.

Yes. That is completely natural.

• Com a chuva, o trânsito fica mais lento.
• O trânsito fica mais lento com a chuva.

Both mean essentially the same thing.

The difference is mainly emphasis:

• Starting with Com a chuva highlights the condition first.
• Starting with O trânsito puts the topic, traffic, first.

Portuguese allows this kind of flexibility quite easily.

A helpful approximation is TRAN-zee-too, but with an important nasal sound in the first syllable.

A few points:

• trân has a nasal vowel
• the stress is on trân
• s between vowels sounds like z, so -sito sounds like -zito

So trânsito sounds roughly like TRAN-zhee-too or TRAN-zee-too, depending on accent, but the nasal vowel is important.

Also note the written accent: â shows the stressed vowel quality in trân.

Yes. Brazilians might say several similar things depending on style and context:

• Quando chove, o trânsito fica pior. = When it rains, traffic gets worse.
• Com chuva, o trânsito fica lento. = With rain, traffic gets slow.
• Na chuva, o trânsito complica. = In the rain, traffic gets complicated.
• Quando está chovendo, o trânsito anda mais devagar. = When it is raining, traffic moves more slowly.

The original sentence is very natural and clear, but these alternatives are also useful because they show common patterns:

• quando chove
• ficar + adjective
• andar + adverb