하다 is an infinitive form for verbs and adjectives

한다 is written form of speech (books, articles etc) for verb 나는 학교로 간다 - I go to school

But! In this written form adjectives are used as 하다

예: 문제는 복잡하다 - The problem is compicated

One more time - ~ㄴ다 is written form and it rarely used in spoken language (usually as a kind of exclamation: 맛있다! 신기하다! Etc.)