Learn Polish: Zyski i odsetki wypłacono wczoraj. - Profits and interest were paid out yesterday.

The -no ending creates an impersonal past tense. It means that "someone" performed the action, but it doesn't specify who. While it translates well to the English passive voice ("were paid out"), it actually acts more like an active verb with an invisible, anonymous subject.

How do we know whether to use -no or -to for this impersonal past tense?

It depends on the infinitive of the verb. Verbs ending in -ać, -eć, and -ić/-yć (like wypłacić) take the -no suffix. You only use -to for verbs ending in a consonant or -nąć, for example, zamknięto from zamknąć (to close).

Are Zyski i odsetki the subject of this sentence since they come first?

No, they are actually the direct object! Because impersonal verbs like wypłacono imply an unspecified subject ("someone paid out"), the things being paid out take the accusative case. For these inanimate plural nouns, the accusative form happens to look exactly like the nominative form.

Is the word odsetki (interest) always plural in Polish?

Yes, when referring to financial interest on a loan or profit, it is always plural. The singular form odsetek does exist, but it means "a percentage" or "a proportion" of something, not financial interest.