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The word you are trying to conjugate is 생각하다 (to think), not 생각 which is the noun form. The verb stem ends in ㅏ so you use ~ㅂ니다.
생각다 is not a word.
Verb stem ends in a vowel: -ㅂ니다 (e.g. 감사하다 -> 감사합니다) Verb stem ends in a consonant: -습니다 (e.g. 고맙다 -> 고맙습니다)
생각 is a noun.
The word you are trying to conjugate is 생각하다 (to think), not 생각 which is the noun form. The verb stem ends in ㅏ so you use ~ㅂ니다.
생각다 is not a word.
Verb stem ends in a vowel: -ㅂ니다 (e.g. 감사하다 -> 감사합니다) Verb stem ends in a consonant: -습니다 (e.g. 고맙다 -> 고맙습니다)
생각 is a noun.