안습니다 => 않습니다, 안입니다 => 아닙니다 These are negative sentence ending forms. You need to understand them as part of such ending forms. * verb sentence ending forms for positive sentences (for present tense). (가다 - base form) -> 갑니다(formal, polite), 간다(formal, plain), 가요(informal, polite), 가(informal, plain). * adjective sentence ending forms for positive sentences. (크다) -> 큽니다, 크다, 커요, 커. * ending forms for the special adjective 이다 ("is" as a equator, as in "AA is BB"). (이다) -> 입니다, 이다, 이에요/예요, (이)야. * sentences ending forms for negative sentences (both verbs and adjectives). (가지 않다) -> 가지 않습니다, 가지 않다, 가지 않아요, 가지 않아. * a special negator adjective 아니다 (is not, negates 이다) (아니다) -> 아닙니다, 아니다, 아니에요, 아니야. * a special negator 않다, 아니하다 = does not. (negates all verbs and adjectives except 이다) (않다) -> 않습니다, 않다, 않아요, 않아. 이다 is the special adjective (technically a particle) corresponding to the coupling "is" as in "AA is BB", where both AA and BB are both noun equivalents. 아니다 negates 이다, and 아닙니다 is its present tense formal and respectful form. 않다 (short for 아니하다) is the special negating verb for all verbs and adjectives except 이다. 않습니다 is the formal and respectful version of 아니하다. Examples: - 나는 학교에 갑니다 <-> 나는 학교에 가지 않습니다 (verb: 가다 <-> 가지 않다). - 나는 키가 큽니다 <-> 나는 키가 크지 않습니다 (adjective: 크다 <-> 크지 않다). - 나는 학생입니다 <-> 나는 학생이 아닙니다. (special particle 이다 <-> ~(이/가/은/는) 아니다). Most grammar books and sites should have detailed explanations of these.