that
这是个强调句.强调句型是：It is/was+被强调部分+that+非被强调部分
原句是：The bell didn't stopped until it was the thirteen stroke.当强调not until句型中的until 部分时,not要同until一起拿到前面去被强调：
It was not until the thirteen stroke that the bell stopped.
强调的是时间状语until...