надо (or нужно) isn't declined for case, it's a part of a compound predicate (alone with an infinitive by which it's followed): Мне надо/нужно написать письмо -- I need/have to write a letter Мне надо/нужно идти -- I have to go. Надо/нужно are also frequently used in impersonal sentences: It's necessary to learn grammar -- Нужно/надо учить грамматику This is something that needed to be don -- Это то, что нужно/надо сделать

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In addition to Denis. Historically, there was a verb “надобеть”. Nowadays, its clipped form is used in Russian language only. However, it can also be used as a separate predicate governing Dat. (who has a need) and Gen. (what is needed), for example, «Ему надо денег. (He needs money.)» or «Ему надо время. (He needs time.)».

надо/нужно it is a predicate noun

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