This time the meaning was translated. Other times the text is translated, resulting in English sentences native English speakers do not use. I am please with the guessing game. This course needs to be reviewed for consistency. (There was a sentence translated to "I like exercise", which is a technically correct translation but incorrect because people use a verb over a noun there - being generous with technocal correctness because it is missing an article to mean an activity, without an article it means exercise as a concept, which is wrong since the Korean sentence does not use it that way.)
In Korean plurals aren't as important when talking about objects (things that aren't people). It's kind of implied if you just say 개 and aren't referring to a specific dog (you'd know in context). When you talk about people though you should use 들. The plurals aren't as important as in English.
나 is "never with formal speech (합니다)"? The tips and notes section disagrees with you:
Can you say 나 in 합쇼체 (-ㅂ니다)?
Definitely yes. 저 is for lowering oneself, and -ㅂ니다 is for raising the listener. If you are higher than the listener, you can raise them by using -ㅂ니다, but you don't have to lower yourself. On the other hand, it is weird to lower yourself and at the same time not raise the listener.
Because using 가 and 는 would make it specific, when in this case it is talking about a general group. 를 always implies it as an object.
Jeo-neun not CHUH-NUN. (Romanizations make it really weird, as the j is not always pronounced like ch) And because it would be weird to raise yourself up, in the notes it says 저 would raise yourself up, but it has a formal ending to address the other person and still make it formal.