Jasminn . I don't know these sentences are OK or not. 'Must' is wrong: it suggests an internal compulsion.
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Jasminn. I don't know these sentences are OK or not.'Must' is wrong: it suggests an internal compulsion. Your other structure is OK, but not quite the native choice:
JasminnSo you mean these are grammatical but natives don't say like this?They don't commonly say so; otherwise, they are OK.
JasminnI'm wondering it's possible to use "under" in this position?Yes. Attractions commonly state "Children under 6 must be accompanied by adults'. Note that 'must' here (and it is the affirmative