Need 这个词可以当作情态动词和实义动词.
当它做情态动词时候,没有时态和人称的变化,其后面接动词原型,如你给的例句；I need do my homework now.它的否定形式为 need not(或者needn't),表示为"没有必要",用于回答 must (必须)的提问.
当它做实义动词时候,有时态和人称的变化,其后面接不定式或者动名词,如你给的例句：I need to do my homework now,它的否定形式为 don't need .如I常don't need to do my homework now.