Must not and Need not are easily confused words. To understand the difference between must not and need not, you must first know their meanings.
The word need can be used both as a verb and as a noun.
(i) Meanings of need as a verb: require; want; call for; necessitate
(ii) (a) Meanings of need as a noun: essential; necessity; requirement
(b)Need as a noun can also mean: poverty; hardship; penury; destitution
The word must can be used both as a verb and a noun.
Meanings of must as a verb: have to; be obliged to; ought to; should
Meanings of must as a noun: obligation; duty; requisite
Now that you know the meanings, you can see the examples below to understand the correct usage of must not and need not.
Examples of Must Not:
1. Narayan's father told him that he must not stay out late and should come home by 10p.m.
2. The Municipal Corporation officials warned the hotel authorities that they must not build another floor as it was against the FSI rules.
3. The doctor told his patient "You must not take an overdose of sleeping pills".
Examples of Need Not:
1. The boss told the office peon that he need not wait till closing time and could leave early.
2. The electrician told Jaydev that he need not install a separate board for connecting the music system.
3. The film director called up the cast & crew and told them they need not come the next day for the outdoor shoot as it had been cancelled due to heavy rains.