I’m encountering more complicated sentences now and I didn’t understand why this verb, to learn, is not conjugated. Is it because it’s a casual sentence and not a polite one? I think “to learn” would be conjugated into naraimasu for a polite sentence, but there are other ways it could be written as well, ne?
Yes, it’s plain form. Casual. Probably talking to a friend. Sara is definitely a friend, given the lack of honorific.
That’s precisely it. (Though “To read (et cetera)” is an awkward translation. Makes it sound like a dictionary entry or something. Even with context, I probably wouldn’t render it like that.)
Thank you 
I would argue the Japanese also sounds like a dictionary entry, that’s why it should be translated as „to read“ or „to eat“. It doesn’t feel like a sentence you would say out lout in Japanese, but a sentence created to practice grammar.
It‘s missing the て-form in most cases (or た-form would also work in the examples)
I don’t think it’s a context problem. Even without context, you can still decide to simply translate something as first person with an implied “I” and that should be fine.