Technically, 이다/아니다 are not verb. 아니다 is adjective and 이다 is particle(은/는/이/가/에게/을/를...those are particle). It is easier on you if you understand them as verb in English. Because verb and adjective can be predicate in Korean. + (There are lots of particle in Korean, but only) '이다' can be predicate. ex: 나는 한국사람이다. 나는 학생이다. I just don't want you to be confused. Lots of Korean grammar book explains like V/A-지만: in this case you can use Verb and Adjective), V-(으)ㄹ 수 있다/없다. : in this case you can use only Verb.