Learn Hardcore Finnish: Ystäväni toimii vapaaehtoisena paikallisessa kirjastossa. - My friend works as a volunteer in the local library.

Ystäväni toimii … → the verb toimii is singular → “My friend works …”
Ystäväni toimivat … → the verb toimivat is plural → “My friends work …”

So in your sentence, because the verb is singular (toimii), ystäväni means “my friend” (one person).

Why is it ystäväni and not minun ystäväni for “my friend”? Are both correct?

Both are correct, but they feel a bit different:

ystäväni = my friend
minun ystäväni = my friend (with extra emphasis on “my”)

In standard Finnish:

You can use just the possessive suffix:
- Ystäväni toimii vapaaehtoisena…
Or you can use both the pronoun and the suffix (often slightly more emphatic or formal):
- Minun ystäväni toimii vapaaehtoisena…

Using only minun ystävä without -ni is seen as colloquial and not standard in writing, although it’s common in spoken Finnish.

Why is the verb toimii used instead of on (is)? What does toimia really mean here?

Toimia literally means “to function”, “to operate”, “to act”, and in this kind of sentence it means “to work (as)” / “to serve (as)”.

Ystäväni on vapaaehtoinen = My friend is a volunteer. (simple statement of identity)
Ystäväni toimii vapaaehtoisena = My friend works/serves as a volunteer. (focus on their role/activity)

With toimia, Finnish typically uses the essive case (-na/-nä) for the role:

toimia lääkärinä = to work as a doctor
toimia opettajana = to work as a teacher
toimia vapaaehtoisena = to work as a volunteer

So toimii vapaaehtoisena emphasizes the function / role rather than just “being” a volunteer.

What case is vapaaehtoisena, and why do we use that form instead of just vapaaehtoinen?

vapaaehtoisena is in the essive case (ending -na / -nä).

Basic form: vapaaehtoinen = volunteer
Essive: vapaaehtoisena = “as a volunteer”, “in the role of a volunteer”

The essive case is commonly used:

to express a temporary role or state
after verbs like toimia and työskennellä (to work)

Examples:

Hän toimii lääkärinä. = He/She works as a doctor.
Olen töissä opettajana. = I work as a teacher.
Ystäväni toimii vapaaehtoisena. = My friend works as a volunteer.

So vapaaehtoisena is required by this pattern: toimia + essive (role).

Both paikallisessa and kirjastossa end in -ssa. Are they the same case, and why do both words get it?

Yes, both are in the inessive case (-ssa / -ssä), which often means “in / at”.

paikallinen = local
paikallisessa = in the local (something)
kirjasto = library
kirjastossa = in the library

In Finnish, adjectives agree in case and number with the noun they modify. That’s why both words take a case ending:

paikallisessa kirjastossa
- paikallisessa (inessive, singular)
- kirjastossa (inessive, singular)

Literally: “in the local library” (local + library both in the same case).
You can’t say *paikallinen kirjastossa if you mean “in the local library”; the adjective has to match: paikallisessa kirjastossa.

What is the difference between toimii vapaaehtoisena and työskentelee vapaaehtoisena?

Both can be translated as “works as a volunteer”, but there is a nuance:

toimii vapaaehtoisena
- Slightly more general.
- Emphasizes functioning / acting in that role.
- Very natural for voluntary or role-based activities.
työskentelee vapaaehtoisena
- Uses the verb työskennellä = “to work (at a job)”.
- Emphasizes the working activity itself.
- Also correct and common.

In your sentence, toimii vapaaehtoisena sounds completely idiomatic and perhaps a bit more neutral/natural when talking about volunteering.

How flexible is the word order here? Can I say Ystäväni toimii paikallisessa kirjastossa vapaaehtoisena instead?

Yes, you can. Finnish has fairly flexible word order, and all of these are grammatical:

Ystäväni toimii vapaaehtoisena paikallisessa kirjastossa.
Ystäväni toimii paikallisessa kirjastossa vapaaehtoisena.

The main difference is emphasis:

Version 1 (original): slight focus on the role (vapaaehtoisena) first, then where it happens.
Version 2: slight focus on where (paikallisessa kirjastossa) first, then what role.

In everyday speech, both sound natural; the meaning is the same.

How would I say “My friends work as volunteers at the local library” in Finnish?

You need to make the verb and role plural:

Ystäväni toimivat vapaaehtoisina paikallisessa kirjastossa.

Breakdown:

Ystäväni = my friend(s) → here, context + toimivat (plural verb) = my friends
toimivat = they work / function (3rd person plural of toimia)
vapaaehtoisina = as volunteers (plural essive of vapaaehtoinen)
paikallisessa kirjastossa = in the local library

So singular vs plural:

Ystäväni toimii vapaaehtoisena… = My friend works as a volunteer…
Ystäväni toimivat vapaaehtoisina… = My friends work as volunteers…

Can you break down each word into its base form and meaning?

Yes:

Ystäväni
- Base: ystävä = friend
- Suffix: -ni = my
  → my friend
toimii
- Base verb: toimia = to function, act, work (as)
- Form: 3rd person singular present (hän toimii = he/she works)
  → works / acts / serves
vapaaehtoisena
- Base: vapaaehtoinen = volunteer (also “voluntary”)
- Case: essive (-na) = as (in the role of)
  → as a volunteer
paikallisessa
- Base: paikallinen = local
- Case: inessive (-ssa) = in / at
  → in the local …
kirjastossa
- Base: kirjasto = library
- Case: inessive (-ssa) = in / at
  → in the library

Full idea: My friend works as a volunteer in the local library.

Finnish has no articles. How do we know if this is “a local library” or “the local library”?

Finnish does not use articles like a/an or the. The sentence:

paikallisessa kirjastossa

can mean:

“in a local library”
“in the local library”

The language usually relies on context to show whether something is new/indefinite or known/definite.

In isolation, a natural English translation would be
“My friend works as a volunteer at a local library.”
In a context where both speakers already know which library is meant, it would be
“My friend works as a volunteer at the local library.”

The Finnish form itself doesn’t change; it’s the same paikallisessa kirjastossa in both cases.