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I know the verb, NEED can be used both as an ordianry verb and a modal verb. Are there any difference between the pairs below:

(a) You don't need to do that.
(b) You needn't do that.

(c) You needn't have done it.
(d) You didn't need to do it.

And plus, do you (native speakers of the English Language) REALLY use the verb NEED as a modal verb in everyday langauage? Such as "Need I do it?" I read that the usuage of the modal verb NEED is a bit archaic and too formal. But I heard that "Need I say more?" is commonly used.

Is my understanding right?

Desperately need your intuition and insight.

Thanks for taking time to read my post.

Thanks in advance.

Jay from ROK
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1. No difference in meaning in the pairs you cite.
2. "a bit archaic" - I agree.
3. "Need I say more?" This, and possibly a few others with modal "need", have become fossilized in the language. They are fixed expressions, or may as well be considered as such, at least in AmE.
4. Your understanding is right.
5. You're welcome!

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Comments  
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needn't =don't need to
didn't need -something wasn't necessary and I knew ,so I didn't that I didn't need to go to the pharmacy because I still had pills.
needn't+perfect infinitive- something wasn't necessary,but I did it I needn't have gone to the meeting.Nothing new came up.
need can be used both as an ordinary verb and a modal verb.
"Need" means something like "must have".
"Need to" is something like "must". It's not quite the same, but it has a very similar meaning.
True: "Need I say more?" is commonly used. Therefore, "yes" to "Do you (native speakers of the English Language) REALLY use the verb NEED as a modal verb in everyday langauage?". But "yes" to rather archaic and formal, even in UK.

(a) and (b) might therefore be synonymous but probably in current usage mean (a) you have no need that is fulfilled by doing that.
(b) nobody sees you as having an obligation to do that.

No time to write more - contact me via (URL removed by mod; we don't like adverts, here.) if you really are desperate.
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