Final answer:
The missing word is 'should'. It emphasizes the importance of refraining from making noise when others are sleeping.
Step-by-step explanation:
In this sentence, the missing word is 'should'. The sentence should read 'You should make noise when others are sleeping'. This sentence expresses an expectation or a suggestion, indicating what is recommended or necessary under a specific condition. By using the word 'should', it emphasizes the importance of refraining from making noise when others are sleeping.
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