Learn Swedish: Jag läste protokollet och agendan i höstas. - I read the minutes and the agenda last autumn.

To talk about a season in the past, Swedish adds the suffix -as to the noun and puts i in front of it. So höst (autumn) becomes i höstas (last autumn). You will see this same pattern for days of the week, like i måndags (last Monday).

Could I use i höstas with a future tense verb?

No, the i ... -as structure is strictly used for the past tense. Because the action already happened, the verb here is läste (read, past tense). If you wanted to say "this autumn" or "next autumn" (looking to the future), you would use i höst or till hösten.

Why is protokollet singular when the English translation says "the minutes"?

In English, "the minutes" (the official written record of a meeting) is a plural noun. In Swedish, this concept is usually singular: ett protokoll. Since it is an ett-word, the definite form ("the minutes") becomes protokollet.

Why does agendan end in -n instead of -en for the definite form?

Agenda is an en-word, but because its base form already ends in a vowel, it only takes an -n to become the definite form agendan (the agenda) rather than adding a second vowel.