PREP
（与否定词连用）直到…之前，直到…为止，除非
You use until with a negative to emphasize the moment in time after which the rest of your statement becomes true, or the condition which would make it true.
例句
The traffic laws don't take effect until the end of the year...
交通法要到年底才生效。
It was not until 1911 that the first of the vitamins was identified.
直到1911年才发现了第一种维生素。