The English modal verb must and its German counterpart muss are cognates—they share the same origin and similar meanings. But when they are negated, they behave in different ways.
- English Barbara must not go means that it is required that Barbara does not go.
- On the other hand, German Barbara muss nicht gehen means that Barbara is not required to go.
To say that Barbara is not required to go, English uses a different modal verb: Barbara need not go. And to say that it is required that Barbara does not go, German also uses a different modal verb: Barbara darf nicht gehen.
One way to describe these facts is by using logicians’ and linguists’ concept of scope:
- In Barbara is not required to go, the negator not takes scope over the modal form required to.
- Conversely, in it is required that Barbara does not go, the modal required that takes scope over the negator not.
To show scope relations concisely and clearly, it is common to use the symbols ¬ for negation and □ for necessity. Using these symbols, the sequence □¬ means it is required that not. Conversely, the sequence ¬□ means it is not required that (and is permitted).
The following table summarises the use of English must and German muss when they are negated:
| Meaning | Logical notation | English | German |
| required that.not | □¬ | must not | darf nicht |
| not.required that | ¬□ | need not | muss nicht |
Spelling German ‘muss’
When I learnt German, the spelling of muss used the ‘scharfes S’ / ‘Eszett’ symbol as follows: muß. The ‘scharfes S’ / ‘Eszett’ (ß) was a compound letter consisting of an s (‘es’) followed by a z (‘zett’). A spelling reform begun in 1996 (and completed in 2006) replaced Eszett with a double s after short vowels, such as /u/ in the word muss.
For an example of Eszett in 19th century English, please see https://languagemiscellany.com/2021/11/an-old-way-of-writing-double-s/
You don’t need to write ‘need not‘
As I wrote at https://languagemiscellany.com/2021/08/you-need-not-understand/ the construction need not is becoming a little old fashioned. Non-native readers of English often find it hard to understand. So if I’m writing for non-native readers, I now use only an alternative version: does not need to.