It seems like you already know the answer! That's great. Need can be both an ordinary verb (ex:she needs money) and a modal(ex: You needn't fill in a form. Need I fill in a form?) A modal is followed by an infinitive without to, in this case, fill. Resource: Practical English Grammar by Michael Swan

"Need" is a so-called semi-modal verb. It can be used modally in questions and negatives -- "Need I go? You need not go." -- but not affirmatively -- "He needs to go". Other verbs somewhat like this are "ought" and "dare".

need is not a modal need is a main verb we add (s) for verbs if the adjective was singular... she needS some milk.. but if the adjective was I or plural (no S) I need some milk They need some milk. I hope this was helpful.

Yes, "she needs some milk." But if it is a question, then "does she need some milk吗？" 'She' is singular, hence the singular form of the verb, which is 'needs'.

thanks,but " You needn’t stay long. ", " Need we leave so soon?" ,the modal form exists