"He needs to know everything!"
Translation:Йому треба знати все!
Hm, it doesn't really depend on the English translation ('need'), it depends on the Ukrainian word you use. For example, you could say «він потребу́є» 'he needs/lacks/demands/requires', and then you'd use the nominative. But потребува́ти doesn't sound well in this sentence.
Ukrainian has a special category of words called state-category words (слова́ катего́рії ста́ну). Those words are used for describing environment or things that don’t depend on us, e.g.:
- Тре́ба = It’s neccessary
- Спеко́тно = It’s hot
- Відо́мо = It’s known
Those can be used with nouns in dative case to express that the state is affecting someone:
- Їй тре́ба = She needs (literally: to her, it’s neccessary)
- Йому́ спеко́тно = He is feeling hot (literally: to him, it is hot)
- Окса́ні відо́мо = It’s known to Oksana
Those are not verbs. They are often translated with verbs or with adjectives into English, but those are neither. Grammatically, some of them look like adverbs, some look like adjectives, while others look like nouns.
If you need to use them in the future or past tense, you need to add the verb 'to be' in the relevant tense:
- Їй було́ тре́ба = She needed
- Йому́ буде́ спеко́тно = He will feel hot
These grammatical structures are due to the fact that Ukrainian has 7 cases, whereas English has only two for nouns (nominative and genetive/ possessive) and three for pronouns (nominative, genitive and accusative/objective). So such words should be interpreted as appropriate in English. There is no direct equivalent. For instance, треба can also be interpreted as "must" in the present tense. Btw, Duolingo uses "жарко" instead of "спекотно" even though the former is a russism.