莫不 is a literary double-negative pattern meaning 'there is no one who doesn't' — i.e., 'everyone does' or 'all are'. It appears in formal written Chinese, often introducing a result or reaction that no person present can avoid. In some classical contexts it can also mean 'perhaps' or 'could it be that', overlapping with 莫非, but the double-negative sense is the primary HSK 7 usage. When learning this word, recognize it as a fixed phrase: 莫 (none) + 不 (not) → 'none not', and the scope is typically universal.