Used to embed a yes/no question within a sentence. Follows the plain form of verbs, adjectives, or nouns.
I don't know whether he will come or not.
Please check whether that is true.
かどうか is the everyday way to express 'whether or not', while か否か is a more formal and literary equivalent used in writing.
かどうか is the fixed phrase for 'whether or not', while か alone can also simply list alternatives (AかBか) or mark a direct question.
Combination of the question particle か, the quotative particle と, and the adverbial particle か, forming a fixed expression for embedded yes/no questions.