Not until yesterdayhad he changed his mind.
This development，coming as it did when the bottom had fallen
该句型的基本用法为：主语+had hardly done...
when+主语+did...。主句用过去完成时，从句用一般过去时第一句是昨天为止，而不是过去的某个点，是昨天之前的时间段，他从来不知道他这么能说，所以用过去完成时
这是倒装句，强调yesterdan，倒回去是He had not changed change his mind until yesterday.这个句子时态要用过去完成时，所以用had.如果你要区分did和had在这类题型中的不同，建议你把2种时态的运用和表达弄清楚.
