Šalili smo se o poslu.

Se is a reflexive particle that belongs with the verb šaliti se, which means to joke or to kid around.

So the basic dictionary form is:

• šaliti se = to joke

You usually cannot just drop se here.
Without it, šaliti would not mean the same thing in normal usage.

So:

• Šalili smo se = we were joking / we joked

In Serbian, the auxiliary smo is a clitic, and clitics usually come in the second position of the sentence or clause.

That is why Serbian often places the main past participle first and then the auxiliary:

• Šalili smo se

This word order is very natural and standard.

You may also see word order change in longer sentences, but the clitic still tends to stay near the beginning:

• Juče smo se šalili o poslu.

So smo is not randomly placed; it follows Serbian clitic-position rules.

It is the past tense.

Serbian past tense is usually made with:

• a past participle
• plus a present-tense form of biti (to be)

Here:

• šalili = past participle
• smo = we are/have auxiliary form used for the past
• se = reflexive particle

So literally, it is built like:

• šalili smo se = we joked / we were joking

Depending on context, English may translate it as either we joked or we were joking.

The form šalili agrees with the subject in number and sometimes gender.

For we, the past participle changes like this:

• šalili smo se = we joked, if the group is all male or mixed
• šalile smo se = we joked, if the group is all female

For singular:

• šalio se = he was joking
• šalila se = she was joking

So šalili tells you the subject is we, and probably a male or mixed group.

Poslu is in the locative case.

The preposition o often means about, and when it has that meaning, it normally takes the locative:

• o poslu = about work / about the job

The noun is:

• posao = work, job

Its locative singular is:

• poslu

So:

• o + locative
• o poslu = about work

Because posao changes form in the locative singular.

This is part of Serbian noun declension.
The noun posao is a masculine noun, and its forms include:

• nominative: posao
• genitive: posla
• dative: poslu
• accusative: posao
• vocative: posle or posao in limited usage, though not very important here
• locative: poslu
• instrumental: poslom

So after o, you need the locative:

• o poslu

This is something learners usually just have to get used to through exposure and practice.

It can mean either, depending on context.

Posao can mean:

• work in a general sense
• a job
• business/affairs in some contexts

So Šalili smo se o poslu could mean:

• we were joking about work
• we were joking about the job

English chooses the most natural version based on context.

Yes, but it means something different.

• Šalili smo se o poslu = we were joking about work
• Šalili smo se na poslu = we were joking at work

So:

• o + locative = about
• na + locative = at/on

That is a very important difference.

Yes, o + locative is a normal and understandable way to express joke about something.

So you can say things like:

• Šalili smo se o politici. = We were joking about politics.
• Šalili su se o školi. = They were joking about school.

However, Serbian can also express similar ideas in other ways depending on nuance, such as joking at someone’s expense or making fun of something. But for a learner, šaliti se o + locative is a good and useful pattern.

A rough pronunciation guide is:

• SHAH-lee-lee smo se oh POH-sloo

A few details:

• Š sounds like sh in shoe
• č / ć do not appear here
• lj in šalili is a soft ly-like sound to many English ears, though here it is just l, not lj
• o is a clear short o, not a diphthong like in many English accents
• u in poslu sounds like oo in food, but shorter and tenser

Serbian pronunciation is usually much more phonetic than English spelling.

A useful breakdown is:

• Šalili = joked / were joking
• smo = we (auxiliary for past tense)
• se = reflexive particle
• o poslu = about work

So the structure is roughly:

• [past participle] + [auxiliary] + [se] + [prepositional phrase]

In more natural English-oriented terms:

• We were joking about work.

But Serbian does not have to copy English word order.

Yes. Serbian usually drops subject pronouns when they are already clear from the verb form.

So you normally say:

• Šalili smo se o poslu.

You could add mi for emphasis or contrast:

• Mi smo se šalili o poslu, a oni o školi.
• We were joking about work, and they were joking about school.

But without emphasis, leaving out mi is perfectly normal.

The verb šaliti se is imperfective.

That means it often describes:

• an ongoing action
• repeated action
• general activity

So Šalili smo se often feels like:

• we were joking
• we spent some time joking

If you want, English may still translate it simply as we joked, but grammatically the Serbian verb itself is imperfective.

Sometimes it can come close, but šaliti se o poslu is usually softer and broader than make fun of.

It most naturally means:

• to joke about work
• to talk humorously about work

If you specifically want make fun of, Serbian often uses stronger or more targeted expressions depending on what exactly you mean.

So the safest understanding here is:

• We were joking about work.

A very useful pattern is:

• šaliti se o + locative

Examples:

• Šalili smo se o poslu. = We were joking about work.
• Šalila se o profesoru. = She was joking about the professor.
• Šalite se o svemu. = You joke about everything.

You can also learn the past tense pattern:

• past participle + auxiliary + se

For example:

• Smejali smo se. = We laughed.
• Dogovorili smo se. = We agreed.
• Vratili smo se. = We returned.

So this sentence is a good model for both past tense and reflexive verbs.