Learn Hardcore Finnish: Ystäväni on muusikko, joka soittaa kitaraa ravintolassa. - My friend is a musician who plays the guitar in a restaurant.

Both of those are correct and mean the same thing. The version with only the suffix (ystäväni) is shorter and very common, especially when the possessor is obvious from context.

Minun ystävä without -ni sounds colloquial or dialectal and is not recommended in formal or learner Finnish.

Can ystäväni also mean my friends?

Yes, ystäväni can mean both my friend and my friends. The form itself does not show singular vs plural.

You understand the number from context, especially from the verb:

Ystäväni on muusikko.
→ on (3rd person singular) → my friend (one person)
Ystäväni ovat muusikoita.
→ ovat (3rd person plural) → my friends (several people)

You can make the plural more visible by adding the plural marker -t before -ni:

ystäväni (sg or pl, depending on context)
ystäväni vs ystäväni are written the same; grammar around them disambiguates.
More explicitly: ystävät (friends) → ystävä
- t
  - ni → ystäväni (friends-my)

So number is read from the sentence, not from ystäväni alone.

Why is muusikko in the basic form after on?

Muusikko is in the nominative (dictionary form) because it is a predicative noun after the verb olla (to be).

In Finnish, when you say someone is something (a profession, a role, an identity), you usually put that noun in the basic form:

Hän on opettaja. = He/She is a teacher.
Isäni on lääkäri. = My father is a doctor.
Ystäväni on muusikko. = My friend is a musician.

There is no separate word for a/an or the in Finnish, so muusikko here means a musician (or a/the musician, depending on context).

What is the difference between on muusikko and on muusikkona?

Both are correct but they have different nuances.

on muusikko (nominative)
- States what someone is by identity or profession.
- More neutral, permanent-sounding.
- Ystäväni on muusikko. = My friend is a musician (that is his/her profession or identity).
on muusikkona (essive case, -na)
- Means as a musician, in the role of a musician.
- Often sounds more temporary, situational, or role-focused.
- Hän on töissä muusikkona. = He/She works as a musician.
- Hän oli mukana projektissa muusikkona. = He/She took part in the project as a musician.

In your sentence, on muusikko is the natural choice because it simply states the person’s profession.

What does joka do in this sentence, and why not kuka?

Joka is a relative pronoun meaning who / that / which. It introduces a relative clause that describes a noun:

Ystäväni on muusikko, joka soittaa kitaraa ravintolassa.
= My friend is a musician who plays the guitar in a restaurant.

Here:

joka refers back to muusikko (or more loosely to ystäväni, since they are the same person).
It is the subject of the clause joka soittaa kitaraa ravintolassa.

Kuka is a question word meaning who?:

Kuka hän on? = Who is he/she?

You use:

kuka in questions.
joka in relative clauses (the one who…, that…, which…).

So kuka would be wrong here; you must use joka.

Why is there a comma before joka in Finnish?

In Finnish, you almost always put a comma before a joka-clause that describes a noun.

Ystäväni on muusikko, joka soittaa kitaraa ravintolassa.

This is just a standard punctuation rule:

A joka-clause is seen as a separate clause attached to a noun, and Finnish writing convention separates it with a comma.

Unlike English, Finnish does not distinguish the comma usage based on restrictive vs nonrestrictive meaning. You still use the comma:

Mies, joka seisoo tuolla, on veljeni.
Auto, joka on pihalla, on minun.

So even if in English you might sometimes omit the comma, in Finnish you generally keep it.

Why is kitaraa in the partitive case and not kitara or kitaran?

Kitaraa is the partitive singular of kitara (guitar).

With the verb soittaa meaning to play (an instrument), the instrument is regularly in the partitive:

soittaa kitaraa = to play (the) guitar
soittaa pianoa = to play (the) piano
soittaa viulua = to play (the) violin

This is a fixed, idiomatic pattern in Finnish. Reasons:

Soittaa + partitive often expresses an ongoing activity rather than a completed, bounded action.
Musical-instrument objects are almost always partitive in neutral statements.

Using kitara (nominative) or kitaran (genitive) would sound wrong or at least very odd in this meaning. You might see kitaran in other structures:

soitin kitaran rikki = I broke the guitar (by playing it)
…but that is a different meaning: kitaran is a fully affected object, not “the instrument I play” in general.

Can we change the word order inside joka soittaa kitaraa ravintolassa, like joka soittaa ravintolassa kitaraa?

Yes, Finnish word order is fairly flexible, especially inside clauses. Both are grammatical:

joka soittaa kitaraa ravintolassa (neutral, common)
joka soittaa ravintolassa kitaraa (possible, but with slightly different emphasis)

Default neutral order often is:

verb – object – place
→ soittaa kitaraa ravintolassa

If you move ravintolassa earlier:

soittaa ravintolassa kitaraa can give more emphasis to the location (“plays in a restaurant (not somewhere else)”).

Meaning stays essentially the same, but:

joka soittaa kitaraa ravintolassa is the most natural, textbook-like version here.

What does the ending -ssa in ravintolassa mean, and how would the meaning change with -lla?

Ravintolassa is:

ravintola = restaurant
-ssa (inessive case) = in, inside

So ravintolassa = in a/the restaurant, typically also understood as at a restaurant in this context.

If you used -lla (adessive):

ravintolalla would mean something like at the restaurant (as a point or area), more “at the place” than “inside the building”.
It is much less natural here for somebody playing music; you’d normally say ravintolassa.

Typical patterns:

olen ravintolassa = I am in/at the restaurant.
olen torilla = I am at the market square.
olen koulussa = I am at school (literally in school).

So for playing guitar as a gig in a restaurant, ravintolassa is the normal choice.

How would the meaning or emphasis change if we said Ystäväni, joka soittaa kitaraa ravintolassa, on muusikko instead?

Both sentences contain the same information, but the focus is different.

Original:
- Ystäväni on muusikko, joka soittaa kitaraa ravintolassa.
  Focus:
- First: My friend is a musician.
- Then extra info: that musician plays guitar in a restaurant.
Changed order:
- Ystäväni, joka soittaa kitaraa ravintolassa, on muusikko.
  Focus:
- First: My friend, who plays guitar in a restaurant (we identify which friend)
- Then: is a musician.

Subtle difference:

In the original, being a musician is the main point; playing in a restaurant is a detail about that musician.
In the second, we start by specifying which friend (the one who plays in a restaurant), and only then state that this person is a musician.

Both are grammatical and natural; choice depends on what you want to emphasize first.

Is it possible to say Ystäväni on muusikko ja soittaa kitaraa ravintolassa instead of using joka?

Yes, you can say:

Ystäväni on muusikko ja soittaa kitaraa ravintolassa.
= My friend is a musician and plays the guitar in a restaurant.

Differences:

With joka:
- Ystäväni on muusikko, joka soittaa kitaraa ravintolassa.
- Shows clearly that the same person is a musician and plays in a restaurant.
- The playing-in-a-restaurant part is grammatically tied to muusikko as a description (a musician who…).
With ja:
- Two coordinated clauses sharing the same subject (ystäväni):
  - My friend is a musician
  - (and) my friend plays the guitar in a restaurant.
- Still usually understood as the same person, but grammatically looser: being a musician and playing in a restaurant are just two separate facts about your friend.

In everyday speech, both versions are fine. The joka version feels a bit more “structured” or written; the ja version a bit more conversational.