"I don't need to cook"
is it not okay so say "Sihitajipiki"? because in negative form we should remove the "ku" and change the last vowel from an A to an I right? Or am I mistaken?
no, the negative form is in the '
sihitaji', because what's being refuted is the need to cook, not the action itself. For sentences that have 'need', 'like', 'want' or 'must' etc., usually their Swahili counterparts will have the negative form, and the verbs remain unchanged e.g.
a kukimbia (you like to run) -
i kukimbia (you don't like to run
ia (you are running) -
ii (you are not running/you don't run)
a kugoma (they want to go on strike) -
i kugoma (they don't want to go on strike)
Sharti/lazima uhudhurie (you must attend) -
Si sharti/lazima uhudhurie (you don't have to attend)