"He thinks that it is a tasty pizza."
Translation:Він думає, що це смачна піца.
What makes you think so?
Having Ukrainian as my mother language, makes me think so. Думати is just like міркувати or розмірковувати - is to think about something. Like: "He thinks about what pizza to order." Or: "He is thinking about a homework problem." Вважати or рахувати - is used when you think something has a certain property/you are trying to express an opinion. "I think this pizza is tasty." Or - "I think Ukrainian is a beatiful language." If you really want you can use думати everywhere, but to me as a native speaker it doesn't sound right.
Having Ukrainian as my mother language, makes me think so.
The course authors, who added this sentence, also had Ukrainian as their mother tongue, so this is hardly an argument. I myself consider Ukrainian to be one of my native languages too (although this depends on the definition of 'native language').
Also, the dictionary written by native speakers also notes this meaning of «думати»:
\3. з спол. що. Мати певну думку, вважати, допускати. Як хто кого боїться, То й думає, що на того ввесь світ Так буде, як і він, дивиться (Леонід Глібов, Вибр., 1957, 48); — Я жадала б пояснень. Інакше я можу думати, що ви просто пожартували зі мною (Юрій Смолич, I, 1958, 51);
So I belive that your usage of «ду́мати» is impossible only in your personal idiolect, or in your dialect, or in some other variety of Ukrainian you speak, but not in the literary Ukrainian.
I probably didn't express myself correctly. I don't think that using the word ду́мати is "impossible" to mean вважати. I was pointing out that in this sentence the verb вважати is more organic.
There is a reason why in the dictionary, you provided, first interpretation of the the word is Розмірковувати над - "think about". That is the main usage of the word. Also note that is says - Мати певну думку, вважати, допускати. The last word meaning - assume.
Now to the usage in poetry in the example you provided. It is clear that in those cases думав combines both "assume" (допускати) and "contemplate" (міркувати) meanings. So I think it is totaly appropriate. (Not that I would question Lesia Ukrayinka or Ivan Franko, to be honest)
Turning back to the translation in question. One doesn't really "contemplate" or "assume" pizza is tasty. He or she merely expresses their opinion on the fact, so I retain that the verb вважати is more appropriate.
To conclude: it is possible to use a word ду́мати with що, to mean "think that", but why would you use it as your primary translation? Just to be easier on people who learn the language? I would use it as an acceptable translation, but not as a primary one. This applies to any language, words often have very particular meanings attached to them, which makes them especially beautiful.
I totally agree that other words sound better in this context.