Here's how I understand it. "I need not your help". WRONG. Old verb negation form. "need not" can only work on another verb. "She needs not worry about that." WRONG. It should be "need", a modal verb which never changes form. "They needn't have left so early in the morning". CORRECT. Using "need" as a modal verb (sounds more formal). "You need not protect me all the time". CORRECT. Same as above. * "needn't have V-ed" and "didn't need to V" might have a slight difference, as in the former implies they did actually V and are now regretting it, while the latter does not suggest this as clearly (i.e. it just says there was no need to do it and nothing more). But I doubt this distinction is recognized in real usage. Anyway, the rule is, 1. in a plain indicative statement, there is no variation. It is always "I need to V". 2. in a negative or interrogative clause, there can be "need (not) V" (modal) and "do/does/did (not) need to" (regular). For example, 1. She needs to see a doctor / She needed to see a doctor / She will need to see a doctor. 2A. Need she see a doctor? / (no modal verb version possible for past or future tense *). 2B. Does she need to see a doctor? / Did she need to see a doctor? / Will she need to see a doctor? * "Need she have seen a doctor?" would mean "She didn't see a doctor, but should she have?", which is different in meaning (same point as was mentioned earlier).