Bibliotekar je ozbiljan.

• bibliotekar = librarian
• je = is
• ozbiljan = serious

So the structure is very close to English: Librarian is serious → more naturally in English, The librarian is serious or A librarian is serious, depending on context.

Serbian normally does not use articles like English the and a/an.

So bibliotekar can mean:

• a librarian
• the librarian

The exact meaning depends on context.

That is very normal in Serbian. Learners often want to insert an article, but you usually simply do not.

Je is the 3rd person singular present form of the verb biti (to be).

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In Bibliotekar je ozbiljan, je links the subject and the adjective, just like is in English:

• The librarian is serious.

In Serbian, adjectives must agree with the noun in:

• gender
• number
• sometimes case

Here, bibliotekar is a masculine singular noun in the nominative, so the adjective also appears in the masculine singular nominative form:

• masculine: ozbiljan
• feminine: ozbiljna
• neuter: ozbiljno

So:

• Bibliotekar je ozbiljan. = The male librarian is serious.
• Bibliotekarka je ozbiljna. = The female librarian is serious.

Both can matter, but here they line up.

Bibliotekar is a grammatically masculine noun, and in normal usage it usually refers to a male librarian. That is why the adjective is ozbiljan.

If you mean a female librarian, Serbian commonly uses:

• bibliotekarka

Then the sentence becomes:

• Bibliotekarka je ozbiljna.

So yes, the adjective changes because the noun changes gender.

It is in the nominative case because it is the subject of the sentence.

In basic equational sentences like this one:

• Bibliotekar je ozbiljan. the subject is nominative, and the predicate adjective also matches that nominative form.

So this is a very standard nominative sentence pattern.

Yes. Serbian word order is more flexible than English, although some orders sound more neutral than others.

The most neutral version is:

• Bibliotekar je ozbiljan.

You may also hear:

• Ozbiljan je bibliotekar.
• Bibliotekar ozbiljan je. — possible in poetry or marked style, but not the usual everyday order

Changing the order often changes emphasis, not the basic meaning.

A learner should usually stick with:

• Subject + je + adjective

In a full standard sentence like this, no, you normally keep it:

• Bibliotekar je ozbiljan.

Leaving it out would sound incomplete or nonstandard in ordinary prose:

• Bibliotekar ozbiljan. ❌

However, in headlines, notes, or very informal shorthand, Serbian may sometimes drop parts that would normally be present. But for normal speaking and writing, keep je.

A rough English-friendly guide:

• Bibliotekar ≈ bee-blee-oh-TEH-kar
• je ≈ yeh
• ozbiljan ≈ oz-BEE-lyan

A few helpful notes:

• j in Serbian is pronounced like English y in yes
• lj is a special sound, roughly like the lli in million for many English speakers
• stress is not usually marked in normal writing, so learners typically pick it up by listening

It most commonly means serious, but like English serious, it can have slightly different shades depending on context, such as:

• not joking
• stern
• thoughtful
• grave
• important

In Bibliotekar je ozbiljan, the most natural reading is simply:

• The librarian is serious

Depending on context, it could imply personality, expression, or attitude.

By itself, it could be either. Because Serbian has no articles, the sentence is more context-dependent than English.

It can mean:

• The librarian is serious.
• A librarian is serious.

In real conversation, context usually makes it clear whether the speaker means:

• a specific known librarian, or
• a librarian being introduced into the conversation

You would change both the noun and the verb/adjective agreement:

• Bibliotekari su ozbiljni. = The librarians are serious.

Changes:

• bibliotekar → bibliotekari (plural)
• je → su (are)
• ozbiljan → ozbiljni (masculine plural)

This is another good example of how Serbian words change to match one another.