Learn Russian: После третьей попытки учёному удалось доказать свою теорию. - After the third attempt, the scientist managed to prove his theory.

The verb удаться (to manage / to succeed) is an impersonal verb. This means the sentence has no grammatical subject performing the action. When there is no grammatical subject, past tense verbs default to the neuter singular form, which is why we see удалось.

Why is "the scientist" translated as учёному in the Dative case?

In Russian, you don't say "I managed." Instead, the construction works more like "It succeeded to me." The person who achieves the success must be in the Dative case when using the verb удаваться / удаться. Here, учёному is the Dative receiver of the success.

Wait, isn't учёный an adjective? Why is it acting as a noun here?

Учёный is a word that started as an adjective (meaning "learned" or "educated") but is now commonly used as a noun meaning "scientist" or "scholar." Even though it acts as a noun, it still uses adjective endings, which is why its Dative form is учёному instead of taking a standard noun ending.

Why do we use свою instead of его for "his" theory?