Learn Hardcore Russian: Учёный нашёл источник реки в горах. - The scientist found the source of the river in the mountains.

Although учёный can function as an adjective meaning “learned,” in this sentence it’s a noun meaning “scientist.” You can tell because it’s the subject (the one doing the action), and it appears in the nominative case (no ending change from its dictionary form).

Why is нашёл spelled with ё, and how is it pronounced?

In Russian, the letter ё always represents the sound [o] with the stress on that syllable. So нашёл is pronounced [nɐˈʂol], with the stress on -шёл. The ё tells you both the sound and that this is the past tense, masculine singular form of найти (“to find”).

Why is источник in the accusative case, and how can I recognize that?

Источник is the direct object of нашёл (he found what?), so it takes the accusative case. Since источник is an inanimate masculine noun ending in a consonant, its nominative and accusative forms look identical. You identify the accusative here because it answers “what?” and follows the verb.

Why is реки in the genitive case?

Реки is the genitive singular of река (“river”). It answers the question “whose source?” or “source of what?” In Russian, when you say “source of the river,” the word “river” goes into the genitive.