Lan bơi ở hồ bơi.

A natural word-by-word breakdown is:

• Lan = a female name
• bơi = to swim
• ở = at / in
• hồ bơi = swimming pool

So the sentence structure is basically:

• Lan
  • swims
    • at/in the swimming pool

They are not doing the same job, even though they look the same.

• The first bơi is a verb: to swim
• The second bơi is part of the noun hồ bơi: swimming pool

So:

• bơi = swim
• hồ bơi = pool

This kind of repetition is completely normal in Vietnamese.

In this sentence, ở marks location. It often means:

• at
• in
• sometimes to stay/live at, depending on context

Here, it connects the action to the place:

• bơi ở hồ bơi = swim at the pool / swim in the swimming pool

English may choose at or in, but Vietnamese uses ở very naturally for location in general.

Vietnamese often follows a straightforward pattern:

• Subject + Verb + Location

So:

• Lan = subject
• bơi = verb
• ở hồ bơi = location phrase

This is similar to English:

• Lan swims at the pool

That makes the sentence quite simple and typical.

Here, Lan is simply a person’s name, most likely a woman’s or girl’s name.

Vietnamese names are not marked with capitals in speech, of course, but in writing, the capital L shows it is a proper name.

A learner should understand this sentence as talking about a person named Lan, not the common noun meaning of any other word.

Vietnamese does not use articles the same way English does.

English usually needs:

• a swimming pool
• the swimming pool

Vietnamese often leaves that information unstated unless it matters.

So hồ bơi can mean:

• a swimming pool
• the swimming pool
• just swimming pool in a general sense

Context tells you which one is meant.

By itself, it can often be understood as either, depending on context.

Vietnamese does not always mark tense the way English does. A simple sentence like this can describe:

• a general fact or habit: Lan swims at the pool
• a current action, if the context supports it: Lan is swimming at the pool

If you want to make the ongoing action clearer, Vietnamese often adds markers such as đang:

• Lan đang bơi ở hồ bơi. = Lan is swimming at the pool.

Yes, you often can.

There is a small difference in emphasis:

• ở hồ bơi focuses on the location: at the swimming pool
• trong hồ bơi focuses more on being inside the pool: in the swimming pool

So:

• Lan bơi ở hồ bơi = Lan swims at the pool
• Lan bơi trong hồ bơi = Lan swims in the pool

Both are natural, but they are not exactly identical.

It is a compound noun:

• hồ = lake / pond / pool
• bơi = swim

Together, hồ bơi means swimming pool.

Vietnamese often creates compound nouns this way, by putting words together very directly.

Vietnamese tones are very important, because changing a tone can change the word.

A rough guide:

• Lan — level tone
• bơi — tone on ơi that should be pronounced correctly; it is not the same as a flat English syllable
• ở — has its own tone
• hồ — low/falling-type tone
• bơi — same as before

If you are learning Northern Vietnamese pronunciation, it helps to listen carefully to a native speaker and repeat the whole sentence as a rhythm unit:

• Lan | bơi | ở hồ bơi

One especially useful thing to notice is that bơi and hồ bơi should sound clearly different in function, even though the word bơi is the same in both places.

No. This sentence is complete as it is.

• Lan is already the subject
• bơi is the verb
• ở hồ bơi gives the location

No classifier is needed before hồ bơi here, because you are not counting it or specifically describing it with a numeral.

For example, a classifier might appear in a different sentence such as:

• một cái hồ bơi = one swimming pool / a swimming pool

But in Lan bơi ở hồ bơi, just hồ bơi is perfectly normal.

No, it sounds natural.

English learners sometimes notice the repetition because in English, swim and swimming pool are closely related too. Vietnamese speakers do not usually find this awkward.

In fact, it is a very ordinary, clear sentence.