Learn Hardcore Croatian: On više ne radi nedjeljom.

nedjeljom (instrumental, no preposition) means “on Sundays” in a habitual/generic sense.
u nedjelju (accusative with the preposition u) means “on Sunday” referring to one specific Sunday (this coming Sunday, last Sunday, etc.). Examples:
On više ne radi nedjeljom. = He no longer works on Sundays (habitually).
On ne radi u nedjelju. = He isn’t working on Sunday (a specific Sunday).

What case is nedjeljom, and why is there no preposition?

It’s the instrumental singular used adverbially to express regular time. Days of the week commonly do this without a preposition:

ponedjeljkom, utorkom, srijedom, četvrtkom, petkom, subotom, nedjeljom = on Mondays, Tuesdays, Wednesdays, Thursdays, Fridays, Saturdays, Sundays (habitually).

Can I drop On?

Yes. Croatian typically drops subject pronouns when the verb ending makes the subject clear:

Više ne radi nedjeljom. = He/She/It no longer works on Sundays. You keep On to clarify it’s a man, for emphasis, or to contrast with someone else.

Can I change the word order?

Yes, Croatian word order is flexible. All of these are acceptable, with small differences in emphasis:

On više ne radi nedjeljom. (neutral)
On ne radi više nedjeljom. (slight emphasis on “not anymore”)
Nedjeljom on više ne radi. (emphasis on “on Sundays”) Avoid splitting ne from its verb: On ne više radi nedjeljom is wrong. Use ne radi više instead.

Is there any difference between više ne and ne … više?

Both mean “no longer / not … anymore.”

više ne + verb is very common: On više ne radi nedjeljom.
ne + verb + više is also fine: On ne radi više nedjeljom. They’re near-equivalent; positioning can slightly shift emphasis in speech.

What does više mean here? Isn’t više “more”?

With negation, više means “anymore / any longer”: Ne radi više. = He doesn’t work anymore.
In affirmative, više means “more”: Radi više. = He works more. To say “still,” use još or još uvijek: On još uvijek radi nedjeljom.

How would I say “He stopped working on Sundays” (focus on the act of stopping)?

Use prestati + infinitive:

Prestao je raditi nedjeljom. Your original sentence states the current situation (he no longer works Sundays) without focusing on the moment of stopping.

Could ne radi mean “is not working (right now)” or even “is broken”?

Yes. Croatian present can be habitual or ongoing:

On ne radi. = He is not working (now) / He doesn’t work.
Stroj ne radi. = The machine doesn’t work / is out of order. Adding nedjeljom removes that ambiguity by making it habitual.

Should nedjeljom be capitalized?

No. Days of the week are written in lowercase in Croatian unless they start a sentence: nedjeljom, ponedjeljkom, …

Why nedjeljom and not nedeljom?

It’s a standard-variety difference:

Standard Croatian: nedjeljom (ijekavian).
Standard Serbian (ekavian): nedeljom. Both mean the same; use the form that matches the standard you’re learning.

How do I say it for other subjects (she/they)?

She: Ona više ne radi nedjeljom.
They (mixed/masc.): Oni više ne rade nedjeljom.
They (all-feminine): One više ne rade nedjeljom. Note the verb endings: radi (he/she/it), rade (they).

Can ne contract with the verb (like one word)?

Generally no: write ne radi as two words. There is contraction with the verb je (to be): ne + je = nije, but not with radi.

How can I say “not at all anymore” for extra emphasis?

Add uopće:

On više uopće ne radi nedjeljom.
On uopće više ne radi nedjeljom. Both are idiomatic; uopće strengthens the negation.

Could I use svake nedjelje instead of nedjeljom?

Yes, but it changes the nuance:

On više ne radi nedjeljom. = He doesn’t work on Sundays anymore (at all).
On više ne radi svake nedjelje. = He no longer works every Sunday (implies he might still work on some Sundays). So use nedjeljom for a blanket statement, svake nedjelje for “every Sunday” specifically.

Is nedjeljama (instrumental plural) possible?

You may see nedjeljama used adverbially (“on Sundays”), but it’s much less common than nedjeljom. In everyday Croatian, nedjeljom is the natural choice.