"What do they think?"

Translation: Τι νομίζουν αυτοί;

[jurgen_maes]

Τι νομίζουν; should be correct too.

[NikosPainnessi]

absolutely

[uSCF10]

βασικά η προσωπική αντωνυμία χρησιμοποιείται μόνο για έμφαση και όχι σε απλές προτάσεις όπως εδώ

[JacobPast177]

What's wrong with Τι σκέφτονται; ?

[Lng52-._]

That's what I would have used as well.

[uSCF10]

Τίποτα :))

[Jon345104]

Would τι αυτοί νομίζουν have the same meaning

[jaye16]

I'm afraid the word order is not correct.

[AniOhevYayin]

To go with jaye16's point: Peter Mackridge (The Modern Greek Language, 237) writes, "clauses introduced by interrogative pronouns, in which the interrogative word is the subject or direct object of the verb, the verb must stand immediately after the interrogative pronoun." You can drop the pronoun, but if you keep it, it must come after the verb.

[PauloMuffato]

What other language(s) do you know follow(s) this same rule?

[CoranceC]

I had assumed that the pronoun was optional in this case, it seems I was mistaken.

[BillDe]

Why is the pronoun needed? (If it is.)

[G.Georgopoulos]

It's not ;)

[jaclynbeierlein]

Why not ποιο?

[D_..]

Ποιο is essentially which one so it doesn't work for What..? in this case.

[ScottHuch]

I answered Τι νομίζούν αυτοί; This was the first hint on hover. But Duolingo told me to pay attention to the accents and said there should only be one accent on νομίζουν, over the iota. I'm not sure which is correct. Reported 5 May 2020.

[D_..]

there should only be one accent on νομίζουν, over the iota.

That is correct.