I can't determine whether to use the definite article-the- or the indefinite article -a- based on correct answers to these exercises.
Can someone help me understand the difference between the use of "요" and "니다" at the ends of sentences/words?
Different speech levels, which require different verb conjugations. See the Formal section for 하십시오체 and Casual section for 해요체.
The first one is for casual form ...and the second one is honorific
회색인 토끼 -> 회색 토끼