Learn Hardcore Finnish: Sähkö katkeaa joskus myöhään illalla. - Electricity sometimes cuts off late at night.

Because sähkö is the grammatical subject of the sentence. In Finnish, subjects take the nominative case. You would only use the partitive (sähköä) if sähkö were a (partial) object or governed by a verb or construction that requires the partitive.

What is the infinitive form of katkeaa, and how do you form its present tense?

The dictionary form is katketa. To make the present tense, you drop -ta from katketa, getting the stem katkea-, and then add the type 1 endings:
– minä katkean
– sinä katkeat
– hän katkeaa
So katkeaa is 3rd person singular present.

Why is the sentence in the present tense even though it describes events that happen at various times (including in the future)?

Finnish often uses the present tense for habitual or regularly recurring actions, and even for future events that are scheduled or expected. Here it means “The electricity goes out (from time to time) late in the evening,” describing a general pattern rather than a single past event.

How would you express a specific past occurrence, for example if the power actually went out last night?

Use the past form of katketa, which is katkesi in 3rd person singular. So you say:
Sähkö katkesi joskus myöhään illalla.

What part of speech is joskus, and can its position in the sentence change?

joskus is an adverb meaning sometimes. Adverbs in Finnish are quite flexible in word order. You can place joskus at the beginning (Joskus sähkö katkeaa myöhään illalla), in the middle (Sähkö joskus katkeaa myöhään illalla), or right before the time expression (Sähkö katkeaa myöhään joskus illalla) with only minor shifts in emphasis.

Why is myöhään used instead of myöhässä or another form?

myöhään is the adverb form of the adjective myöhäinen (late), used to describe when something happens. myöhässä is the essive case (used with olla) meaning “to be late” (for an appointment, for example), so it doesn’t fit here.

Why do we say illalla and not illassa or illalle?

In Finnish, the adessive case (ending -lla/–llä) is used for general time expressions: illalla means “in the evening.”
– illassa (inessive) would literally mean “inside the evening,” which isn’t used for time.
– illalle (allative) would mean “toward the evening,” focusing on arrival, not timing of an event.

What is the difference between illalla and yöllä?

illalla covers evening hours (roughly from late afternoon until about 10 pm). yöllä covers night hours (roughly from 10 pm until early morning). If the power cut happens after midnight, you’d say yöllä instead of illalla.