“de” has a few other uses that don’t translate to “by using”, but they aren’t quite as common and you can usually figure them out on the spot by just looking at the sentence. In the future you might want to revisit “de” and learn a bit more about all the different ways it can be used, but for a beginner this grammar is all you really need.
図書館 = としょかん = library
Blue: Some verbs work best when used with another object.
Yellow: You write using a pen. You study using a library.
Blue: You use で to mark these helper objects and locations.
Blue: In English で usually translates to “with”, but other prepositions are possible.
Yellow: I wrote a letter with a pen. I studied at the library.
Blue: Those were some surprisingly responsible sounding examples.
Yellow: Thanks! I copied them out of your notebook.