U našem odjelu svi rade mirno i bez stresa.

Because u here means in, describing a location, and after u Croatian uses the locative case for location.

So:

• odjel = department
• u odjelu = in the department

The adjective/pronoun naš must agree with odjel in gender, number, and case, so it becomes:

• našem odjelu = in our department

So the whole phrase u našem odjelu is literally in our department in the correct case form.

Odjel usually means department, section, or division within an organization.

Depending on context, it can refer to:

• a company department
• a school department
• a hospital ward

In this sentence, it most naturally means department.

A learner should not confuse it with ured, which more often means office.

Svi means all or everyone, referring to people.

In this sentence, it means everyone in the department.

A useful detail:

• svi = all/everyone, masculine plural form
• Croatian often uses the masculine plural form for a mixed group or an unspecified group of people

So even if the department includes both men and women, svi is normal.

If the group were only women, you could say:

• sve rade = all of them work / all the women work

Because svi is grammatically plural, so the verb must also be plural.

The verb is raditi = to work

Present tense:

• ja radim = I work
• ti radiš = you work
• on/ona radi = he/she works
• oni/one rade = they work

So:

• svi rade = everyone works / all work

This is a point that can feel different from English, because English often says everyone works with a singular-looking verb. In Croatian:

• svi rade = everyone/all are working
• but svatko radi = each person works / everyone works

So svi takes rade, while svatko takes radi.

Because mirno is an adverb, and it describes how people work.

• miran / mirna / mirni = calm, quiet, peaceful (adjectives, describing nouns)
• mirno = calmly, quietly (adverb, describing a verb)

Here it describes rade:

• rade mirno = they work calmly

Compare:

• mirni ljudi = calm people
• ljudi rade mirno = people work calmly

So mirno is correct because it modifies the action, not the people directly.

Because the preposition bez always takes the genitive case.

So:

• stres = stress
• bez stresa = without stress

This is a very common pattern in Croatian:

• bez šećera = without sugar
• bez problema = without problems
• bez vode = without water

So the learner should remember:

• bez + genitive

Yes, they are closely related, but not exactly identical.

• mirno = calmly, peacefully, quietly
• bez stresa = without stress

So the sentence gives two linked ideas:

• the work is done in a calm way
• the work environment is not stressful

In natural English, this could sound like:

• Everyone works calmly and without stress
• Everyone works in a calm, stress-free way

No, Croatian word order is fairly flexible.

U našem odjelu is placed at the beginning to set the scene: in our department.

That makes the sentence sound natural and topic-first:

• U našem odjelu svi rade mirno i bez stresa.

Other word orders are possible, for example:

• Svi u našem odjelu rade mirno i bez stresa.

This still means basically the same thing, but the emphasis shifts slightly toward everyone in our department.

So the original word order is natural, but not the only possible one.

Because Croatian does not have articles like a or the.

English says:

• in our department

Croatian simply says:

• u našem odjelu

The idea of definiteness is understood from context, and in this case našem already makes it specific: it is our department, not just any department.

So Croatian learners should get used to the fact that nouns usually appear without articles.

Yes, but the grammar changes.

You could say:

• U našem odjelu svatko radi mirno i bez stresa.

That means something like each person in our department works calmly and without stress.

The difference is:

• svi rade = everyone/all are working
• svatko radi = each person works

So both are possible, but:

• svi takes a plural verb: rade
• svatko takes a singular verb: radi

That is a very useful contrast for learners.