that引导的宾语从句在什么情况下不能省略that

    在 agree, announce, argue, assume, aver, calculate, reply, object, conceive, contend, hold, indicate, learn, maintain, observe, reckon, remark, state, suggest 等动词之后，引导宾语从句的 that 一般不省略。（摘自《薄冰英语惯用法词典》that 词条）

    当两个或两个以上有 that 引导的从句在句中为并列宾语时，第一个 that 可省略，第二个或以后的 that 则往往不省。第一个 that 引导的宾语从句较长时，尤其如此。例如：Joe said (that) Peter would take them by car, but that they might be late. 乔说彼得将用汽车送他们，但也说他们可能迟到。/ He said he would come, and that he would bring his son. 他说他会来，而且会把他的儿子带来。（同上）