Learn Hardcore Portuguese (Brazil): O parque está mais vazio do que ontem.

O parque = the park (a specific park we both know about).
Just saying parque está mais vazio sounds wrong; you need an article or another determiner (like este parque, aquele parque, meu parque, etc.).

So in this sentence, O parque is the natural way to say “The park”.
If you were speaking in very general terms (parks in general), you might drop the article and say Parques são importantes, but not in this specific-situation sentence.

Why is it está and not é?

Portuguese uses ser and estar differently:

estar = temporary state, condition, location, result of a change
ser = permanent or defining characteristic, identity, time, origin, etc.

O parque está mais vazio do que ontem.
uses estar because the park is emptier right now, as a temporary condition that can change.

If you said O parque é vazio, that would mean the park is inherently empty (like it is always empty, part of its nature), which is a very different idea.

Why do we say mais vazio and not something like “emptier” with a special ending?

Portuguese usually forms comparatives with mais + adjective:

mais vazio = emptier / more empty
mais bonito = prettier / more beautiful
mais caro = more expensive

There is a small set of irregular comparatives (melhor, pior, maior, menor), but for most adjectives you just use mais + adjective.
So mais vazio is the normal, correct comparative form.

Shouldn’t vazio agree with parque? Why isn’t it vazia or vazios?

It does agree.

parque is masculine singular, so the adjective must be masculine singular: vazio.
If it were plural:
- Os parques estão mais vazios do que ontem.
If it were a feminine noun:
- A praça está mais vazia do que ontem.
- As praças estão mais vazias do que ontem.

So the form vazio is exactly what we expect with o parque.

Why is it mais vazio do que and not just mais vazio que?

Both forms are used in Brazilian Portuguese:

mais vazio do que ontem
mais vazio que ontem

In everyday Brazilian speech, both are correct and natural here.
Historically, do que came from a preposition (de) plus que, but for learners it’s easiest to treat mais … do que as one common pattern for comparisons.

You can safely use either mais … do que or mais … que with adjectives, and you’ll hear both all the time.

Is do que here really de + o que (“of the that”)? What does that do mean?

In this specific structure, learners should not try to translate do literally.

In mais vazio do que ontem, the do que is a fixed comparative connector.
It does not mean “of the that”, and there is no article corresponding to anything in English here.

So think of do que as part of the comparison pattern, not as de + o in the usual sense.
You wouldn’t say de o que ontem; the form is just do que.

Could I say O parque está mais vazio que ontem instead? Is that more or less natural?

Yes, you can:

O parque está mais vazio que ontem.
O parque está mais vazio do que ontem.

In Brazilian Portuguese, both are very common and natural.
Some people feel mais … do que is a bit more careful or explicit in certain formal contexts, but in normal speech the difference is minimal. You don’t need to worry about it at beginner or intermediate levels.

Why is it do que ontem and not something like do que ontem estava (“than it was yesterday”)?

Portuguese often leaves out repeated words when they’re obvious from context.

Full version (clear but usually not said):

O parque está mais vazio do que (estava) ontem.

The verb estava is understood; we know it refers to the state of the park yesterday, so it’s simply omitted.
This is similar to English leaving out words like “than (it was) yesterday” where “it was” is also usually dropped.

Why don’t we need any article or preposition before ontem?

Words like ontem (yesterday), hoje (today), amanhã (tomorrow) are adverbs of time, not nouns.

They do not take articles: no o ontem, a hoje, etc. (except in very special, poetic or stylistic uses).
In this sentence, do que already covers the comparative connection, and ontem simply tells us when.

So do que ontem is the normal, simple way to say “than yesterday.”

Can I change the word order, like O parque mais vazio está do que ontem?

No, that word order is wrong in Portuguese.

The natural pattern is:

O parque está mais vazio do que ontem.
[subject] [verb estar] [mais + adjective] [do que + comparison]

You cannot split mais and vazio, and you can’t move está to the end like that.
Word order is more rigid around comparatives than English might feel to you.

Could I drop está and say just O parque mais vazio do que ontem?

No, you need a verb. Portuguese sentences generally require an explicit verb.

O parque está mais vazio do que ontem. ✅
O parque mais vazio do que ontem. ❌ (sounds like a fragment, not a complete sentence)

Without está, it doesn’t form a proper statement, just a noun phrase that feels incomplete.

Could I also say O parque está menos cheio do que ontem? Is there any difference from mais vazio?

Yes, you can say:

O parque está menos cheio do que ontem. = The park is less full than yesterday.
O parque está mais vazio do que ontem. = The park is emptier than yesterday.

In many contexts they mean practically the same thing, but the emphasis is slightly different:

mais vazio highlights emptiness.
menos cheio highlights the reduction in how full it is.

Both are common and correct; which one you choose is more about nuance and personal style than about grammar.