Learn Hardcore Finnish: Hän treenaa kotona juoksemalla olohuoneessa. - He/She trains at home by running in the living room.

Finnish does not mark grammatical gender in third-person singular pronouns.
Context usually makes it clear whether hän refers to a man or a woman, but if needed, you can specify with words like mies (man) or nainen (woman).

So Hän treenaa... can mean He trains... or She trains... depending on context.

Why is the verb treenaa and not something like treenaaN or treenata?

The base verb is treenata (to train, to work out).

treenata is an infinitive form (dictionary form), not used as the finite verb of the sentence.
For hän (he/she), you use the 3rd person singular present tense.

Conjugation (present tense, active) of treenata:

minä treenaan – I train
sinä treenaat – you train
hän treenaa – he/she trains
me treenaamme
te treenaatte
he treenaavat

So treenaa is correct for hän.

You could also use a more “standard” verb:

Hän harjoittelee kotona... – He/She practices at home...
Hän kuntoilee kotona... – He/She works out / exercises at home...

But treenaa is very common in everyday speech.

Why is it kotona and not koti or kotiin?

All three forms are from koti (home), but they express different local cases:

koti – base form: home (no location implied)
kotona – inessive-type form: at home
kotiin – illative: (to) home
kotoa – elative: from home

In this sentence:

kotona = at home, a static location.

So:

Hän on kotona. – He/She is at home.
Hän menee kotiin. – He/She goes home.
Hän lähtee kotoa. – He/She leaves home.

Koti is irregular: instead of kotissa, Finnish uses kotona for at home.

Why is olohuoneessa used for “in the living room,” and why not something like olohuoneessa kotona instead of having both kotona and olohuoneessa?

Olohuone means living room.

olohuoneessa is the inessive case (ending -ssa / -ssä) and means in the living room.

The sentence has two locative expressions:

kotona – at home (general place)
olohuoneessa – in the living room (more specific place within the home)

Together they express:

He/She trains at home, specifically by running in the living room.

It’s natural in Finnish to have both:

Hän treenaa kotona juoksemalla olohuoneessa.
→ He/She trains at home by running in the living room.

You could drop kotona:

Hän treenaa juoksemalla olohuoneessa. – He/She trains by running in the living room.

That is also correct; you just lose the explicit mention of “at home.”

Why does kotona end with -na but olohuoneessa ends with -ssa, even though both mean “at / in” somewhere?

This looks odd because koti is irregular.

Normally, the inessive (meaning in / inside) is formed with -ssa / -ssä:

talossa – in the house
kaupungissa – in the city
olohuoneessa – in the living room

However, koti uses a special locative set:

kotona – at home (functionally like an inessive)
kotiin – (to) home (illative)
kotoa – from home (elative)

So:

olohuoneessa follows the regular -ssa pattern.
kotona is just a lexical irregularity you need to memorize as “at home.”

What exactly is juoksemalla, and how is it formed from the verb juosta?

Juoksemalla is a special non-finite verb form:

It is the 3rd infinitive instructive form of juosta (to run).
It often translates as “by doing X” or “through doing X.”

Formation:

Take the verb stem: juosta → juokse-
Add -malla / -mällä:
- juoksemalla – by running

Function in the sentence:

Hän treenaa kotona juoksemalla olohuoneessa.
→ He/She trains at home by running in the living room.

More examples:

Hän laihtuu syömällä vähemmän. – He/She loses weight by eating less.
Opin parhaiten puhumalla. – I learn best by speaking.

So juoksemalla answers the question “How does he/she train?” – By running.

Could we just say Hän juoksee olohuoneessa instead of using juoksemalla? What’s the difference?

Yes, both are correct, but they mean slightly different things and have different focus:

Hän juoksee olohuoneessa.
– He/She runs in the living room.
→ Simple statement of what is happening.
Hän treenaa kotona juoksemalla olohuoneessa.
– He/She trains at home by running in the living room.
→ Emphasizes that running is a method of training.

In (2), treenaa is the main action, and juoksemalla describes how he/she trains. In (1), juoksee itself is the main action.

Is there any difference between juoksemalla and juosten? Could the sentence use juosten instead?

Juoksemalla and juosten are related but not interchangeable in every context.

juoksemalla – 3rd infinitive instructive, very clearly “by running” (method).
juosten – present active participle in -en form, often expresses manner too, but is stylistically more limited and can sound bookish or idiomatic.

You could say:

Hän treenaa kotona juosten olohuoneessa.

This is understandable, but juoksemalla is more natural here for “by running” as a method of training. Juosten is more often seen in fixed phrases and in more literary styles.

Why is there no comma before juoksemalla in this sentence?

In Finnish, -malla / -mällä constructions like juoksemalla are often treated as closely integrated adverbials of manner (answering “how?”), not as separate clauses.

When short and directly attached to the main verb, they usually do not take a comma:
- Hän laihtui syömällä vähemmän.
- Opin tämän tekemällä paljon harjoituksia.

You might see a comma if the -malla phrase is longer, parenthetical, or contrastive, but in this simple sentence it is natural without a comma:

Hän treenaa kotona juoksemalla olohuoneessa.

Can the word order juoksemalla olohuoneessa be changed? For example, can I say olohuoneessa juoksemalla?

Yes, Finnish word order is relatively flexible, especially with adverbials.

Both are grammatically correct:

Hän treenaa kotona juoksemalla olohuoneessa.
Hän treenaa kotona olohuoneessa juoksemalla.

The differences are mostly about information structure and slight emphasis:

The original version flows as:
- trains at home (how?) by running (where?) in the living room.
Swapping them might slightly emphasize the place (olohuoneessa) before the method (juoksemalla), but in everyday speech the difference is small.

The original order is very natural and common.

Why is there no word for “the” in olohuoneessa (no “the living room” vs “a living room” distinction)?

Finnish has no articles like English a / an / the.

olohuoneessa can mean in a living room or in the living room depending on context.
Here, since we already know it’s his/her home, it is naturally understood as the living room in his/her home.

So:

olohuoneessa – in (the) living room
No additional word is needed to specify a or the. Context does that work.