"A tree needs water."
Translation:Дереву нужна вода.
«Надо» is not used with nouns, only with infinitives.
Just got this wrong literally four times and I still have no clue what's going on :/
нужна - This is the short, feminine, form of нужный. Which is an adjective meaning 'necessary'.
When using this adjective the thing which is in need, the tree, needs to be in the dative. дерево - дереву
Normal caveats of course... I don't actually know if this is correct.
I'm pretty sure the thing which is needed is usually (always?) in the nominative. In the case of 'some water' then it would probably be in the partitive case (воды?).