"The man sends a letter."
Translation:남자가 편지를 보냅니다.
고양이는 사람이 아닙니다. (The cat is not a person)
In this sentence, 고양이 (cat) is the topic of the sentence. 사람 (person) is the subject of the sentence. And 아닙니다 is the verb meaning "is not/are not".
For 고양이 the ending particle "는" is used.
For 사람 the ending particle is "이" is used.
The difference is that 고양이 is the topic and 사람 is the subject.
Hope this helped! :)