Learn Hardcore Vietnamese: Tôi mua một cái áo màu đỏ ở thành phố.

một = one, and often works like English “a / an / one”.
cái = a classifier, a little word used before many countable, inanimate nouns.

So:

một cái áo ≈ one shirt / a shirt
Literally: I buy one CL shirt red color in city.

You normally need a classifier when you:

Count things: một cái áo, hai cái áo (one shirt, two shirts)
Refer to a specific, single item.

For áo (shirt, top), cái is the most common general classifier.

How do I know this sentence is past tense (“I bought”) in Vietnamese?

Vietnamese verbs do not change form for tense. mua can mean:

I buy
I am buying
I bought
I will buy (with other markers)

You get tense/aspect from context or from extra words:

Tôi đã mua một cái áo màu đỏ ở thành phố.
→ đã marks past (“I bought / I have bought”).
Tôi đang mua một cái áo màu đỏ ở thành phố.
→ đang marks progressive (“I am buying”).
Tôi sẽ mua một cái áo màu đỏ ở thành phố.
→ sẽ marks future (“I will buy”).

Your original sentence Tôi mua một cái áo màu đỏ ở thành phố. is neutral; the time is understood from the situation or previous sentences, so in many contexts it’s naturally translated as “I bought…”.

Why is “áo màu đỏ” instead of something like “đỏ áo”? What is the word order for adjectives?

In Vietnamese, the normal order is:

noun + (color / size / adjective)

So:

áo màu đỏ = shirt red-color
nhà to = house big
xe mới = car new

You almost never put the adjective before the noun the way English does. So “đỏ áo” is wrong; it must be “áo đỏ” or “áo màu đỏ”.

What does màu do in “áo màu đỏ”? Is it necessary? Could I just say “áo đỏ”?

màu literally means “color”.

áo màu đỏ = a shirt (with the) red color
áo đỏ = a red shirt

In everyday speech:

Both áo đỏ and áo màu đỏ are correct.
áo đỏ is shorter and very common.
áo màu đỏ sounds slightly more explicit about the color (sometimes used when contrasting colors, describing choices, etc.).

So yes, you can definitely say:

Tôi mua một cái áo đỏ ở thành phố.
→ Totally natural: “I bought a red shirt in the city.”

Can I leave out một or cái? What changes in meaning?

Yes, both can be omitted, but the nuance shifts.

Drop “một” but keep the classifier:
- Tôi mua cái áo màu đỏ ở thành phố.
- Often sounds like you’re talking about a specific shirt that both speaker and listener know about (like “the red shirt”).
Drop the classifier but keep “một”:
- Tôi mua một áo màu đỏ ở thành phố.
- Grammatically possible but sounds unnatural; with một, you normally must use a classifier.
Drop both “một” and “cái”:
- Tôi mua áo màu đỏ ở thành phố.
- More general; could mean you bought that kind of item (red shirts), not stressing “one item”.
- Depending on context, it can be understood as “I bought red shirts” or just “I bought a red shirt” without focusing on number.

The most textbook-neutral version for “I bought a red shirt” is your original:

Tôi mua một cái áo màu đỏ ở thành phố.

Could this sentence also mean “I bought red shirts” (plural)? How do you show plural?

As written:

Tôi mua một cái áo màu đỏ ở thành phố.

The presence of một (= one) and cái (classifier) makes it clearly singular: one shirt.

To express plural, you can use plural markers like:

những (some / several, unspecified)
các (all, a set, more definite group)

Examples:

Tôi mua những cái áo màu đỏ ở thành phố.
→ I bought (some) red shirts in the city.
Tôi mua các cái áo màu đỏ ở thành phố.
→ I bought the red shirts in the city (a particular known group).

Often in casual speech, people just say:

Tôi mua áo màu đỏ ở thành phố.

and rely on context to show whether it’s one or more.

Can I move “ở thành phố” to the beginning of the sentence?

Yes. Vietnamese word order is quite flexible for location phrases. All of these are grammatical:

Tôi mua một cái áo màu đỏ ở thành phố.
Ở thành phố, tôi mua một cái áo màu đỏ.

The meaning is the same. Differences:

Final position (…ở thành phố) is very normal in conversation.
Initial position (Ở thành phố, …) can sound a bit more “story-telling” or written, or it can emphasize the location:
“In the city, I bought a red shirt.”

What’s the difference between ở, tại, and trong for places like “ở thành phố”?

All three relate to location, but they’re used differently:

ở – most common, neutral “at / in”
- ở thành phố = in the city / at the city (general)
- Used in everyday speech.
tại – more formal, often in writing, notices, official context
- tại thành phố = at the city
- You’ll see this in documents, announcements, news, etc.
trong – literally “inside”
- trong thành phố = inside the city (emphasizes being within city limits, not outside)
- Slight nuance of inside vs outside.

So in your sentence, ở thành phố is the most natural conversational choice.

Why is the subject “tôi” used here? Could other pronouns be used instead?

Tôi is a relatively neutral “I” and works well in:

Formal or semi-formal situations
Writing
When you don’t want to mark age/gender/relationship strongly

Vietnamese pronouns depend a lot on who you are and who you’re talking to. Instead of tôi, you might say:

em – if you’re younger than the listener
anh – if you’re a man, older or same age as the listener
chị – if you’re a woman, older or same age as the listener
con – when talking to your parents / much older elders

The rest of the sentence stays the same:

Em mua một cái áo màu đỏ ở thành phố.
Anh mua một cái áo màu đỏ ở thành phố.

The choice of pronoun carries social information, not grammatical tense or person in the English sense.

Can the subject pronoun “tôi” be omitted in real conversation?

Yes, very often.

If it’s clear from context who the subject is, Vietnamese speakers commonly drop pronouns:

(Tôi) mua một cái áo màu đỏ ở thành phố.

In a conversation where it’s obvious that you’re talking about your own actions, people will understand “I bought a red shirt in the city” even without tôi.

However, in isolation (a single sentence with no context), including tôi makes it clearer for learners.

Does “thành phố” mean any city or a specific city? Do I need to name the city?

thành phố literally means “city”.

ở thành phố can mean:
- “in the city” in a general way (as opposed to the countryside), or
- in the (relevant) city already known in the conversation.

To be explicit about which city, you normally add the name:

ở thành phố Hồ Chí Minh (in Ho Chi Minh City)
ở thành phố Hà Nội (in Hanoi)

So:

Tôi mua một cái áo màu đỏ ở thành phố.
→ could be understood as “I bought a red shirt in the city” (general) or “in the city we’ve been talking about”.

How do you pronounce this sentence, and what are the tones on each word?

The sentence:

Tôi mua một cái áo màu đỏ ở thành phố.

Word by word with tones (using tone names):

Tôi – mid-level tone (ngang)
mua – mid-level tone (ngang)
một – heavy low, “glottal stop” tone (nặng)
cái – high rising tone (sắc)
áo – high rising tone (sắc)
màu – low falling tone (huyền)
đỏ – dipping / asking tone (hỏi)
ở – dipping / asking tone (hỏi)
thành – low falling tone (huyền)
phố – high rising tone (sắc)

Very rough pronunciation guide (Southern-ish):

Tôi – “toy”
mua – “mua” (like “mua” in “Kuala Lumpur”; close to “mwa”)
một – “mot” with a short, heavy stop at the end
cái – “gái / guy” with a rising tone
áo – “ow” (like “cow”) with a rising tone
màu – “maow” (like “mao”) with a low, falling tone
đỏ – “dor?” with a questioning contour
ở – like “uh?” with a questioning contour
thành – “tàhn” with a low fall
phố – “foh” with a rising tone

Correct tone is crucial in Vietnamese; changing the tone typically changes the meaning of the word entirely.