Learn Hardcore Portuguese (Portugal): Hoje o trânsito está pior do que ontem. - Today the traffic is worse than yesterday.

O trânsito em Lisboa é mau. – Traffic in Lisbon is bad. (in general, as a characteristic of the city)
Ao fim da tarde o trânsito é sempre pior. – In the late afternoon the traffic is always worse. (a regular pattern, not just today)

Why do we say pior do que and not mais mau do que?

Pior is the irregular comparative form of mau (bad), just like worse is the comparative of bad in English.

mau → pior (bad → worse)

You do not say:

✗ mais mau do que ontem

Correct options:

Hoje o trânsito está pior do que ontem. – Today the traffic is worse than yesterday.
Hoje o trânsito está muito mau. – Today the traffic is very bad. (no comparison)

So:

Use mau when not comparing.
Use pior when comparing (worse).

Can I say pior que ontem instead of pior do que ontem?

Yes, you will hear both:

pior do que ontem
pior que ontem

In European Portuguese, pior do que is more standard and a bit more formal/careful, especially in writing.
Pior que is also used in speech and is not wrong, but pior do que is the “safe” choice in Portugal, especially for learners.

Why does trânsito take the article o, but hoje and ontem don’t take any article?

In Portuguese, definite articles (o, a, os, as) are used more often than in English:

o trânsito – the traffic
a comida – the food
os carros – the cars

Here, o trânsito is a normal noun phrase and the subject of the sentence, so it usually takes an article in Portuguese.

But hoje (today) and ontem (yesterday) are adverbs of time, not nouns:

hoje – today
ontem – yesterday

Adverbs do not take articles:
You never say ✗ o hoje or ✗ o ontem in this meaning.

So:

Hoje o trânsito está pior do que ontem.
- Hoje = adverb (no article)
- o trânsito = noun phrase (takes article)
- ontem = adverb (no article)

Could I drop the article and say Hoje trânsito está pior do que ontem?

No, that is ungrammatical in European Portuguese.

You need the article here:

✅ Hoje o trânsito está pior do que ontem.
✗ Hoje trânsito está pior do que ontem.

In Portuguese, singular countable and many mass nouns usually need an article when they are the subject, much more than in English. English allows “Traffic is bad today”, but Portuguese normally needs:

O trânsito está mau hoje.

Is trânsito countable? Can I say something like “a traffic” or “one traffic” in Portuguese?

No, trânsito works like traffic in English: generally uncountable in this meaning.

You don’t say ✗ um trânsito for “a traffic”.
Typical patterns are:

Há muito trânsito. – There is a lot of traffic.
Há pouco trânsito. – There is little traffic.
O trânsito está intenso. – The traffic is heavy.

So you treat trânsito as an uncountable mass noun, but it still takes the article o when used as a general noun phrase (as in this sentence).

Can I change the word order and say O trânsito hoje está pior do que ontem?

Yes. Both are correct:

Hoje o trânsito está pior do que ontem.
O trânsito hoje está pior do que ontem.

The difference is mainly emphasis:

Starting with Hoje puts more emphasis on “today”, today’s situation.
Putting hoje after o trânsito slightly emphasizes the traffic and then situates it in time.

What you would not say is something like:

✗ O trânsito está pior do que ontem hoje.

Placing hoje at the very end after the comparative sounds unnatural here.

Why is it está (singular) and not estão (plural)?

Because o trânsito is grammatically singular in Portuguese.

Even though in reality traffic involves many cars, the word trânsito itself is a singular mass noun:

O trânsito está mau. – singular
O trânsito está pior do que ontem. – singular

If you changed the subject to a plural noun, the verb would change:

Os carros estão piores hoje. – The cars are worse today.
As estradas estão piores hoje. – The roads are worse today.

Could I use the imperfect estava instead and say Hoje o trânsito estava pior do que ontem?

Yes, but the meaning changes:

Hoje o trânsito está pior do que ontem.
→ You are talking now, in the present moment. You’re currently observing that today’s traffic is worse than yesterday’s.
Hoje o trânsito estava pior do que ontem.
→ You are talking about an earlier moment today, from a later point in time (usually you say this later in the day or another day).
Example: At night, telling someone about your day:
Hoje o trânsito estava pior do que ontem. – Today the traffic was worse than yesterday.

So:

está = current situation
estava = past description of today’s situation

Could I say Hoje há mais trânsito do que ontem instead? What’s the difference?

Yes, that’s another natural sentence:

Hoje há mais trânsito do que ontem.
→ Today there is more traffic than yesterday.

Difference in nuance:

está pior – focuses on how bad the traffic is (quality/intensity). It could be because of more cars, more accidents, slower flow, etc.
há mais trânsito – focuses directly on the quantity of traffic (there are more cars on the road).

Both could be true at the same time, but they highlight different aspects.

Is pior an adjective or an adverb here, and does it change for masculine/feminine?

Here, pior functions as an adjective describing o trânsito.

However, the form pior itself is invariable for gender and number:

O trânsito está pior. – masculine singular
A situação está pior. – feminine singular
Os resultados estão piores. – plural (here you also have a plural form piores)
As coisas estão piores.

In the basic predicate “X está pior”, the word pior doesn’t change between masculine and feminine singular. It only changes to piores for plural subjects if you want agreement:

Os serviços estão piores do que antes.
As estradas estão piores do que antes.

How do you pronounce trânsito and hoje in European Portuguese?

In European Portuguese:

trânsito
- roughly: TRÁN-zi-tu
- trã-: nasal “ã” (like “on” in French bon), stressed
- -zi-: like “zee” but shorter
- -to: the final o is often an unstressed, almost “uh”-like sound [ɨ] or [u], depending on accent
hoje
- roughly: O-zhɨ
- first syllable Ho-: the h is silent, so you start with an o sound
- -je: like the French “je”; a soft zh sound + a very reduced e [ɨ]

So you don’t pronounce the h in hoje, and the final vowels in both words are quite reduced in European Portuguese.