Learn Hardcore Dutch: De stagiair werkt vrijwillig op het kantoor. - The intern works voluntarily at the office.

Stagiair is a de-word (common gender), so you must say de stagiair.
This is not something you can always predict from the form of the word; it’s mostly a matter of vocabulary you have to memorize.
The feminine form stagiaire (with -e) is also a de-word: de stagiaire.

So:

de stagiair = (male or unspecified) intern
de stagiaire = (often female) intern

What is the difference between stagiair and stagiaire?

Both words refer to an intern / trainee and are borrowed from French.

de stagiair: traditionally the masculine form
de stagiaire: traditionally the feminine form

In modern Dutch:

Many people still use stagiair for a man and stagiaire for a woman.
But you will also see stagiair used as a more general word for any intern, regardless of gender.

Grammatically, both take de, never het.

Why is the verb werken changed to werkt here?

The base form (infinitive) is werken (to work). Dutch present tense conjugation for regular verbs:

ik werk – I work
jij / je werkt – you work
hij / zij / het werkt – he / she / it works

De stagiair is third person singular (he/she/it type), so we use the form:

werkt = werk (stem) + t

This is similar to English adding -s:

he works → Dutch: hij werkt

Could the sentence also be De stagiair vrijwillig werkt op het kantoor?

No, that word order is wrong in a main clause.

In Dutch main clauses, the finite verb (the conjugated verb: werkt) must be in second position (V2 rule):

De stagiair – first element
werkt – verb in 2nd position
Rest of the sentence: vrijwillig op het kantoor

So:

✅ De stagiair werkt vrijwillig op het kantoor.
❌ De stagiair vrijwillig werkt op het kantoor.

You can move vrijwillig or op het kantoor around within the part after the verb, but the conjugated verb must stay second.

What exactly does vrijwillig mean here? Does it mean he is a volunteer?

Vrijwillig literally means voluntarily / of one’s own free will.

In this sentence, it can imply:

He works without being forced.
Often also: he works unpaid, like a volunteer.

But be careful:

vrijwillig = voluntary / voluntarily (adverb or adjective)
vrijwilliger = a volunteer (noun, person)

So:

Hij werkt vrijwillig. – He works voluntarily.
Hij is vrijwilliger. – He is a volunteer.

In context, werkt vrijwillig often means he is functioning as a volunteer, but grammatically it describes how he works, not what he is.

Why is it op het kantoor and not in het kantoor?

Both op het kantoor and in het kantoor are possible, but they have different nuances.

op het kantoor is idiomatic and usually means at the office (as a workplace, location in a general sense).
in het kantoor focuses more on being inside the office (room) physically.

Compare:

Hij werkt op het kantoor.
– He works at the office (that’s his workplace).
Hij zit in het kantoor.
– He is in the office (room) (physically located there, could be temporary).

So op kantoor ≈ English at the office.

Why is it het kantoor and not de kantoor?

Kantoor is a het-word (neuter noun), so it takes het:

het kantoor – the office

Like with stagiair, this is mostly vocabulary you need to memorize. There is no simple rule that always tells you de/het; you learn it word by word.

Some patterns (with many exceptions):

Diminutives (-je, -tje, -pje, etc.) are always het: het tafeltje.
Many words borrowed from French ending in -eur or -age tend to be de.

But kantoor: just remember het kantoor.

Can you also say op kantoor without het? What’s the difference?

Yes, both are possible:

op kantoor (without article)
op het kantoor (with article)

op kantoor:

Very common expression meaning at the office / at work in general.
Not focused on a specific building, more on the type of place (workplace).

op het kantoor:

Refers more to a specific office, a particular location already known from context.
For example, if there are several buildings and you are talking about that specific office.

So:

De stagiair werkt vrijwillig op kantoor.
– The intern works voluntarily at the office (in general, at his job).
De stagiair werkt vrijwillig op het kantoor naast het station.
– Here het kantoor is specific: the office next to the station.

Where can vrijwillig go in the sentence? Could I say De stagiair werkt op het kantoor vrijwillig?

Yes, adverbs in Dutch are relatively flexible, as long as the verb stays in second position. All of these are grammatical, with small differences in emphasis:

De stagiair werkt vrijwillig op het kantoor.
(Neutral; vrijwillig describes the manner of working.)
De stagiair werkt op het kantoor vrijwillig.
(Slightly more emphasis on vrijwillig, often in contrast to some other place or situation.)
Vrijwillig werkt de stagiair op het kantoor.
(Marked word order, stressing vrijwillig strongly; this sounds more formal or stylistic.)

The most natural everyday version is the original:

De stagiair werkt vrijwillig op het kantoor.

How do you pronounce stagiair and kantoor?

Approximate pronunciations:

stagiair: /staːˈʒɛr/
- sta- like sta in start (but without the English R)
- -giair has a zh sound (like measure) + air
kantoor: /kɑnˈtoːr/
- kan- like con in contact (short o/a sound)
- -toor like toor in door, but with a long oo /oː/ and a lighter Dutch r

Stress:

sta-GIAIR (stress on the second syllable)
kan-TOOR (stress on the second syllable)