Learn Finnish: Minä opiskelin paljon syksyllä. - I studied a lot in the autumn.

opiskella is a Type 3 verb. To form the past tense, we take the consonant stem (opiskel-), add the past tense marker -i-, and then attach the personal ending for 'I' (-n). This gives us opiskelin.

Wait, doesn't opiskella have two Ls? Why is there only one L in opiskelin?

That is a very common trap! To find the consonant stem of a Type 3 verb, we remove the final -la or -lä from the dictionary form. For opiskella, dropping the -la leaves us with the stem opiskel-. Therefore, only one L remains before we add the -i- and -n.

Why do we use the -llä ending for syksyllä instead of -ssä to mean IN the autumn?

While -ssa/-ssä usually means 'in', Finnish uses the adessive ending -lla/-llä (which often means 'on' or 'at') for seasons of the year. So, 'in the autumn' is translated as syksyllä, just like 'in the summer' would be kesällä.

Is paljon always used like this, or does it change cases?

paljon is an adverb meaning 'a lot' or 'much'. Unlike nouns and adjectives, it does not change its ending based on its grammatical role in the sentence. It stays exactly the same here to describe how much you studied.