Learn Hardcore Norwegian: Ikke bare trener hun, men også barna trener i parken. - Not only does she exercise, but the children also exercise in the park.

Norwegian main clauses follow the V2 rule: the finite verb must be in second position. When you front something (here the adverbial phrase Ikke bare), the verb comes next, then the subject: Ikke bare (1) trener (2) hun (3). Compare:

I dag trener hun.
Aldri har jeg sett noe sånt.
I parken trener barna.

Can I start with the subject instead of using inversion?

If you want to keep the same meaning (adding a second subject), a natural subject-first alternative is to switch to the correlative både … og:

Både hun og barna trener i parken. If you want to keep the “not only … but also” idea while starting with the subject, a cleft is idiomatic:
Det er ikke bare hun som trener; barna trener også i parken.

Where should I put også, and what changes with its position?

Place også next to what is being “added” (its focus).

Subject focus: … men også barna trener i parken. (= the children too)
Clause/predicate focus (neutral): … men barna trener også i parken. (= they also train)
Locative focus: … men barna trener i parken også. (= in the park too, perhaps elsewhere as well) Starting the clause is possible but marked for emphasis: … men også trener barna i parken.

Does i parken apply to both “she” and “the children,” or only the second clause?

As written, i parken naturally attaches to the second clause (the children). To make it clear that both train in the park, rewrite:

Ikke bare trener hun i parken, men barna trener der også.
Både hun og barna trener i parken.

Is the comma before men required?

Yes. You’re joining two main clauses, and Norwegian punctuation places a comma before men in such cases:

Ikke bare trener hun, men også barna trener i parken.

Could I use både … og instead of ikke bare … men også?

Yes. It’s shorter and very natural here:

Både hun og barna trener i parken. Use både … og when you’re simply listing two (or more) parallel elements without the contrastive feel of “not only … but also.”

Why is it barna and not barn?

Because barna is the definite plural (“the children”). The forms of the neuter noun barn are:

et barn (a child)
barnet (the child)
barn (children)
barna (the children)

Why is it i parken (the park) and not “in a park”?

I parken is definite, implying a specific park. If you mean any park, use the indefinite:

i en park = in a park You can also pluralize:
i parker = in parks (indefinite plural)
i parkene = in the parks (definite plural)

What tense is trener? Does it mean “exercises” or “is exercising”?

Norwegian present tense covers both simple and progressive meanings:

Hun trener = “She exercises” / “She is exercising.” To stress an ongoing action, you can say:
Hun holder på å trene.

What’s the difference between å trene and å øve?

å trene: to train/work out; physical exercise, sports practice, or systematic training of skills.
å øve: to practice (especially skills like music, pronunciation, a specific task). Examples:
Hun trener hver dag. (works out)
Han øver på piano. (practices the piano) There’s overlap when talking about skill drills: å trene på uttale (to practice pronunciation) is fine.

Should it be hun or henne here?

Hun is the subject form (she). Henne is the object form (her). In this sentence the pronoun is the subject, so hun is correct.

Subject: Hun trener.
Object: Jeg ser henne. A gender-neutral option sometimes used is hen (subject/object).

Could trener be misunderstood as the noun “trainer/coach”?

In context it’s clearly the verb (present tense of å trene). The noun is en trener (a coach/trainer). Compare:

Verb: Hun trener i parken. (She trains/works out…)
Noun: Hun er trener. (She is a coach.)

Can I omit the second trener to avoid repetition?

You can, but only if you supply a support verb, otherwise it sounds clipped:

Good: Ikke bare trener hun, men også barna gjør det.
Safer (fully parallel): Ikke bare trener hun, men også barna trener. In careful writing, keeping the verb in both clauses is clear and natural.

Can også start the second clause?

Yes, for emphasis, and V2 still applies:

Ikke bare trener hun, men også trener barna i parken. This is grammatical but marked; the more neutral choice is … men barna trener også i parken.

Can I front the place phrase?

Yes. Fronting still triggers V2:

I parken trener ikke bare hun, men også barna.
I parken trener både hun og barna.

Do I need men? What about Ikke bare trener hun, også barna trener i parken?

Use men in this correlative structure. Standard Norwegian pairs ikke bare … men (også) …. Without men, it sounds off:

Preferred: Ikke bare trener hun, men barna trener også i parken.

Where does the main stress/intonation fall?

Typically:

Rising emphasis on ikke bare (especially bare),
Strong stress on the added element (barna),
Final content word (parken) often gets phrase-final stress. So you’d highlight: “Not only trains SHE, but ALSO THE CHILDREN train in the PARK.”