Learn Hardcore Finnish: Maksoin sähkölaskun eilen, jotta sähkö ei katkeaisi. - I paid the electricity bill yesterday so that the electricity wouldn't be cut off.

maksoin is the first-person singular imperfect (simple past) of maksaa (“to pay”). For verbs ending in -aa or -ää, you drop the final -aa, add -oi to the stem, then attach the personal ending -n for minä.
Example: maksaa → maksa- + oi → maksoi + n → maksoin.

Why does sähkölaskun end with -n?

Finnish marks a definite, completed singular direct object with the accusative -n ending. Since you paid the electricity bill (a specific, finished action), sähkölasku (“electricity bill”) becomes sähkölaskun.

Why is eilen (yesterday) used without any case ending, and why is it placed after the object?

eilen is an indeclinable adverb of time (“yesterday”). It doesn’t take case endings. Finnish word order is flexible, so placing a time adverb after the object—maksoin sähkölaskun eilen—is perfectly normal. You could also say Eilen maksoin sähkölaskun or Maksoin eilen sähkölaskun without changing the meaning.

What role does jotta play here, and how does it differ from että?

jotta introduces a purpose clause (“so that …”), expressing intention or goal. että introduces content clauses (e.g. reported speech or thoughts). You cannot use että for purpose.
In this sentence, jotta sähkö ei katkeaisi = “so that the electricity wouldn’t be cut off.”

Why is the verb katkeaisi in the conditional mood rather than the present?

In Finnish, purpose clauses with jotta typically use the conditional to show an intended or potential result. katkeaisi is the conditional form of katketa (“to be cut off”), meaning “would be cut off.” A plain present (katkeaa) wouldn’t convey the same sense of “avoiding” a possible future cut.

Why does the negative auxiliary ei come before katkeaisi, and why is sähkö placed between them?

Finnish negation uses the auxiliary ei, which always precedes the main verb form. When there’s an explicit subject, it normally sits between ei and the inflected verb. Hence the order: sähkö ei katkeaisi (“the electricity would not be cut off”).

Why is the subject pronoun minä omitted before maksoin?

Finnish verb endings encode person and number, so the pronoun minä (“I”) is superfluous and usually dropped. The form maksoin already tells you the action was done by “I.”