Why is "is eating/eats" 먹습니다 but "is drinking/drinks" is 마십니다 and not 마습니다? Is this right?
Because 마시다 ends with a vowel "ㅣ" so you have to add ㅂ니다, and 먹다 ends with the consonant "ㄱ" so you add 습니다
ㅂ니다: When the verb ends in a vowel
습니다: When the verb ends in a consonant