Learn Finnish: Me soudettiin paljon järvellä.

In spoken Finnish, the standard "we" forms (like me soudimme) are rarely used. Instead, speakers use the pronoun me followed by the passive form of the verb. Soudettiin is the past passive of soutaa (to row), so me soudettiin is the natural, everyday way to say "we rowed".

Do I have to include the pronoun me, or can I just start the sentence with soudettiin?

You must include me when you want it to specifically mean "we". If you drop the pronoun and just say soudettiin paljon järvellä, it acts as a true passive sentence, meaning "There was a lot of rowing on the lake" or "People rowed a lot on the lake."

How does the verb soutaa (to row) turn into soudettiin?

To form the past passive for Type 1 verbs, you use the weak grade of the verb stem and add the passive past ending -ttiin. The single -t- in soutaa weakens to -d-, which gives you the stem souda-. Then, replacing the final -a with the ending gives you soudettiin.

Why does "on the lake" use the -llä ending (järvellä) instead of the -ssä ending (järvessä)?