Learn Hardcore Portuguese (Brazil): Eu quero colocar o casaco na cadeira.

Eu quero colocar o casaco na cadeira.
Very common and neutral in Brazil. Colocar literally means “to place / to put.”
Eu quero pôr o casaco na cadeira.
Also correct and a bit shorter. Pôr is a standard verb meaning “to put.” In informal Brazilian Portuguese, many people prefer colocar or botar in everyday speech.
Eu quero botar o casaco na cadeira.
Very informal/regional (more common in some parts of Brazil, like the South and Northeast). In many contexts it just means “to put.”

All three are understandable. If in doubt, colocar is a safe, neutral choice in Brazilian Portuguese.

What exactly does na mean in na cadeira, and why not em a cadeira?

Na is a contraction of:

em (in / on / at)
a (the – feminine singular article)

So:

em + a = na

We never say em a cadeira in normal speech or writing; we must contract it to na cadeira.

Em can often mean both “in” and “on,” so na cadeira is usually translated as on the chair here, but it could also mean in the chair depending on context.

Why do we say na cadeira (“on the chair”) if em often means “in”?

Portuguese doesn’t always separate “in” and “on” as strictly as English does. The preposition em can cover both, and context plus the noun tells you which is meant.

Examples:

na cadeira – on the chair (you put things on a chair)
na caixa – in the box
na mesa – on the table
na sala – in the room

If you really need to stress “on top of,” you can say:

Eu quero colocar o casaco em cima da cadeira. – I want to put the coat on top of the chair.

But na cadeira alone is normally enough.

Why is it o casaco and not just casaco?

Portuguese uses articles (o, a, os, as) more than English does.

o casaco = the coat (masculine, singular)
um casaco = a coat

Leaving out the article (colocar casaco) sounds unnatural here, like broken or telegraphic Portuguese.

Only in some specific contexts (titles, labels, lists, etc.) can you drop the article, e.g. on a sign: Proibido casaco (not a very natural example, but you might see article-less phrases in headlines or labels). In normal sentences, you usually need the article: o casaco or um casaco.

How do I know that casaco is masculine and cadeira is feminine?

You mostly have to memorize the gender with the noun, but there are some tendencies:

Nouns ending in -o are usually masculine:
o casaco, o carro, o livro
Nouns ending in -a are usually feminine:
a cadeira, a casa, a porta

So:

o casaco – masculine → article o
a cadeira – feminine → article a → na cadeira (em + a)

There are exceptions, but this rule works often enough to be useful.

Can I change the word order and say Eu quero colocar na cadeira o casaco?

You can, and it’s grammatically correct, but it’s less natural in everyday speech.

Neutral, most common order:

Eu quero colocar o casaco na cadeira.
Subject (Eu) – verb (quero colocar) – direct object (o casaco) – place (na cadeira)

Eu quero colocar na cadeira o casaco might appear in more formal or literary contexts, or if you want to emphasize na cadeira for some reason, but in normal spoken Brazilian Portuguese, people almost always say it the first way.

How would I say “I want to put it on the chair” when “it” refers to the coat?

There are a few options; some are more formal than others.

More formal / standard:

Eu quero colocá-lo na cadeira.
Here, -lo = him/it (masculine, referring to o casaco).
colocar + o → colocá-lo (with a written accent).

Colloquial Brazilian speech often avoids these attached pronouns and uses ele/ela instead:

Eu quero colocar ele na cadeira.
Quero colocar ele na cadeira. (very common)

In writing (especially formal writing), colocá-lo is preferred. In everyday speech, colocar ele is what you’ll hear most often in Brazil.

Could I say um casaco instead of o casaco?

Yes, but the meaning changes slightly:

Eu quero colocar o casaco na cadeira.
→ some specific coat that both speaker and listener likely know about (“the coat”).
Eu quero colocar um casaco na cadeira.
→ some coat, not a specific one known to both people (“a coat”).

So it’s the same distinction as the vs a in English. Use o for specific, um for non‑specific.

What’s the difference between cadeira and words like assento or banco?

cadeira – a chair (with a back, for one person)
na cadeira = on the chair
assento – “seat” (more abstract: a place where you sit, like on a plane, bus, theater)
assento 12B = seat 12B
banco – can mean “bench” (something long you sit on) or “bank” (financial institution), depending on context.

In this sentence, cadeira is the natural choice: it’s just a normal chair.

How do I pronounce the sentence naturally in Brazilian Portuguese?

Approximate pronunciation (Brazilian):

Eu – like “eh-oo” blended, often close to “ay-o”
quero – KEH-roo
- qu before e = k sound (like “kero”)
- the r in the middle is a soft flap, similar to the American “tt” in “butter”
colocar – co-lo-KAR
- co like “coh”
- stress on the last syllable: co-lo-CAR
o – short “oo” sound, like very short “oh” / “oo”
casaco – ka-ZA-co
- stress in the middle: ca-ZA-co
na – like “nah”
cadeira – ka-DAY-ra
- dei = “day”
- stress in the middle: ca-DEI-ra

Spoken smoothly, it sounds like:
“Eu KEH-ro co-lo-KAR o ka-ZA-co na ka-DEI-ra.”

Is there a difference between Eu quero colocar o casaco na cadeira and Eu vou colocar o casaco na cadeira?

Yes:

Eu quero colocar o casaco na cadeira.
→ I want to put the coat on the chair. (It expresses desire/intention; it may or may not actually happen.)
Eu vou colocar o casaco na cadeira.
→ I am going to put the coat on the chair. (This expresses a near future action or decision; it’s more like a plan or promise.)

So quero = want, vou = I’m going to / I will.