Learn Hardcore Croatian: Stol je opet prljav.

The adjective agrees with the subject in gender and number. Stol is masculine singular, so the predicate adjective must be masculine singular nominative: prljav.

Masculine: prljav — Stol je prljav.
Feminine: prljava — Stolica je prljava.
Neuter: prljavo — More je prljavo.

Can I move opet to other positions?

Yes, with slight changes in emphasis:

Neutral: Stol je opet prljav.
Emphasizing “again”: Opet je stol prljav.
End-focus (more colloquial/surprised): Stol je prljav opet. Do not say: ✗ Stol opet je prljav (the clitic je must be in second position; see next answer).

Why is je in the second position?

Je is a clitic, and Croatian clitics generally occupy the “second position” in the clause (after the first syntactic unit).

Stol je opet prljav. (subject first → clitic second)
Opet je stol prljav. (adverb first → clitic second)
U kuhinji je stol prljav. (prepositional phrase first → clitic second)

Can I omit je, like “Stol opet prljav”?

Normally, no. In standard speech you include je. Omitting it is possible in headlines, notes, or very telegraphic style, but it sounds incomplete in everyday conversation.

Do I need a word for “the” or “a” before stol?

Croatian has no articles. Stol can mean “the table” or “a table” depending on context. If you need to be explicit:

ovaj stol (this table)
taj stol (that table, near the listener)
onaj stol (that table, farther away)

Is stol the same as sto?

In standard Croatian:

stol = table (preferred/standard)
sto = one hundred In Serbian/Bosnian, sto means “table,” and Croatians will understand it, but stol is the Croatian standard. Related words:
stolić = small table/coffee table
radni stol / uredski stol = desk

Why is it stol here and not stolu or stola?

Because stol is the subject in the nominative case. Other cases appear with different roles/prepositions:

Locative: na stolu (on the table)
Accusative: Vidim stol. (I see the table)
Genitive: noga stola (the leg of the table)

What’s the difference between Stol je prljav and prljav stol?

Stol je prljav: a full clause stating a fact (“The table is dirty”).
prljav stol: a noun phrase (“a dirty table”). If you want both ideas (“again” + attributive adjective), you typically keep the sentence form: Stol je opet prljav.

How do I say it in the negative?

Use nije (ne + je):

Stol opet nije prljav. (The table isn’t dirty again.) The negative form nije replaces je (you don’t use both).

How do I say it in the past or future?

Past: Stol je opet bio prljav. (was again dirty)
Future: Stol će opet biti prljav. (will be dirty again) Note the clitic placement: će also sits in second position.

Is opet the only way to say “again”? What about ponovno/ponovo/iznova?

opet: very common, often a bit more colloquial; can imply mild annoyance depending on tone.
ponovno/ponovo: “again/anew,” a touch more neutral/formal; ponovno is stylistically preferred in Croatian, ponovo is also widely used.
iznova: “afresh/from scratch,” emphasizes starting over. All work here: Stol je ponovno prljav.

Could I say Stol je opet prljavo?

No. Prljavo is neuter or an adverbial form. Since stol is masculine, the predicate adjective must be prljav. Prljavo would be correct with a neuter subject: More je prljavo.

Can I front the adjective for emphasis, like Prljav je stol?

Yes. Prljav je stol is grammatical and emphasizes “dirty,” almost like “Dirty is the table.” The clitic je still appears in second position after the first stressed word (Prljav).

Any quick pronunciation tips?

stol: like English “stole” without the e; short o.
je: “yeh.”
opet: stress usually on the first syllable: “OH-pet.”
prljav: the cluster “prl” is tight; lj is a palatal sound, roughly like the “lli” in “million”: “PR-lyav.” Roll the r lightly.