Verb conjugation of "brummen" in German

I ordered a cab at four o'clock in the morning which took me to the Central Bus Station. I was much to early because the bus would go just at seven o'clock. Anyway I was sitting there and listening to music.
After our system update, lessons will be saved for 30 minutes automatically, if you want to save them permanently, you need to press the 'save lesson' button. It than works the same basically.
In order to get to this hidden lake one would have to row beneath a cave. On the other side of the cave the five of us were alone. The lake was surrounded by limestone mountains covered with tropical trees.