Learn Hardcore Finnish: Ystäväni on tällä hetkellä huolissaan kokeesta, vaikka hän on harjoitellut hyvin. - My friend is currently worried about the exam, even though she has studied well.

Ystäväni = ystävä (friend) + -ni (my) → my friend
You can also say minun ystäväni, which literally is my my-friend. This is not wrong; it just adds emphasis: my friend (and not someone else’s).

So:

Neutral, normal: Ystäväni on huolissaan…
Slightly more emphatic: Minun ystäväni on huolissaan…

What exactly is tällä hetkellä, and how is it different from nyt?

Both relate to time, but they’re not identical:

nyt = now, very general.
tällä hetkellä = literally at this moment, often translated as currently / at the moment / right now.

Nuance:

Ystäväni on nyt huolissaan. = My friend is now worried.
Ystäväni on tällä hetkellä huolissaan. = My friend is currently / at the moment worried (suggests a temporary state, contrasts with other times).

Tällä hetkellä is a bit more formal/explicit than nyt and fits well in written language.

What is huolissaan grammatically, and how do I use it?

Huolissaan is part of the fixed expression olla huolissaan jostakin = to be worried about something.

The pattern is:
- olen huolissani – I am worried
- olet huolissasi – you are worried
- hän on huolissaan – he/she is worried
- olemme huolissamme – we are worried
- olette huolissanne – you (pl.) are worried
- he ovat huolissaan – they are worried

In this sentence, hän = 3rd person singular → huolissaan.

You don’t normally change the case of huolissaan itself; you just change the ending according to the person and then add the thing you are worried about in the elative case (here: kokeesta).

Why is it kokeesta and not just koe? What case is this?

Kokeesta is elative case (the “out of / from” case). The base word:

koe = exam, test
kokeesta = from / of / about the exam

Certain expressions require elative:

olla huolissaan jostakin = to be worried about something

So:

huolissaan kokeesta = worried about the exam

Other examples with elative after olla huolissaan:

Olen huolissani sinusta. – I’m worried about you.
He ovat huolissaan rahasta. – They are worried about money.

What does vaikka mean here, and is the comma before it always needed?

In this sentence, vaikka means although / even though:

…, vaikka hän on harjoitellut hyvin.
→ …, although he/she has practised well.

The comma is normal: Finnish usually puts a comma before most subordinate clauses, including those starting with vaikka.

Examples:

Lähden ulos, vaikka sataa. – I’ll go out, although it’s raining.
Hän hymyili, vaikka oli väsynyt. – He/she smiled, although he/she was tired.

So yes, you generally keep the comma before vaikka in standard written Finnish.

Why is it hän on harjoitellut (perfect) and not a simple past like hän harjoitteli?

On harjoitellut is the perfect tense: has practised.

Nuance:

hän harjoitteli – he/she practised (simple past, focuses on the action in the past, finished).
hän on harjoitellut – he/she has practised (links the past practice to the present moment; result is still relevant now).

In this sentence, we want to say:

Right now the friend is worried, even though up to now they have practised well.

That “up to now, and it matters now” feeling fits the perfect tense on harjoitellut better than simple past harjoitteli.

What is the difference between harjoitella and opiskella?

Both can relate to learning, but they focus on slightly different things:

harjoitella = to practise, to rehearse, to drill a skill
- harjoitella pianoa – practise piano
- harjoitella kielioppia – practise grammar
opiskella = to study (more general, often formal/academic)
- opiskella matematiikkaa – study mathematics
- opiskella yliopistossa – study at university

For an exam, both can appear, but they emphasize different aspects:

hän on harjoitellut hyvin – he/she has practised well (e.g. done lots of exercises, mock tests).
hän on opiskellut hyvin – he/she has studied well (more general: has been diligent with their studies).

Here, harjoitellut suggests active practice for the exam.

Why is it hyvin and not hyvä?

Hyvä is an adjective = good.
Hyvin is the corresponding adverb = well.

In English you also say:

He practised well (not practised good).

Similarly in Finnish:

hyvä harjoitus – a good practice (adjective before a noun)
harjoitella hyvin – to practise well (adverb after a verb)

So on harjoitellut hyvin = has practised well.

Can I change the word order, for example Tällä hetkellä ystäväni on huolissaan kokeesta?

Yes. Finnish word order is relatively flexible, and you can move elements to emphasize them.

All of these are grammatical, with slightly different emphasis:

Ystäväni on tällä hetkellä huolissaan kokeesta.
→ Neutral focus on my friend; tällä hetkellä sits in the middle.
Tällä hetkellä ystäväni on huolissaan kokeesta.
→ Emphasis on right now; good if you contrast with other times.
Ystäväni on huolissaan kokeesta tällä hetkellä.
→ Slightly stresses about the exam right now; still fine.

The original order is very natural, but rearranging like this does not change the basic meaning.

Could I drop hän and say …, vaikka on harjoitellut hyvin?

In everyday spoken Finnish, yes, you often hear:

…, vaikka on harjoitellut hyvin.

The subject hän is understood from context.

In standard written Finnish, it is more typical (and clearer) to keep the subject pronoun:

…, vaikka hän on harjoitellut hyvin.

So:

Spoken / informal: omitting hän is common if the subject is clear.
Formal / careful writing: prefer …, vaikka hän on harjoitellut hyvin.