Mother told Tom to watch the milk until it boiled and then--

这是tell sb to do A and do B 的结构 told 之后有两个不定式短语做宾补to watch the milk until it boiledto turn off the gas当两个不定式被并列连词and 等连起来时第二个不定式常省去to所以此空为动词原形  请在客户端右上角评价点“满意”即可,