英语语法问题（关于代词）one of the most important questions they had to

您好.这题that和what在语法上都没有错,只是意义上的区别.
一、
1）one of the most important questions they had to consider was （that）of public health
其中they had to consider 为定语从句,修饰先行词questions,省略了作宾语的关系代词（that、which）,即questions （that、which）they had to consider .
2）例句中的that,不是引导从句的,是代词,指代one question,
那么,这句话翻译为：他们不得不考虑的重要问题中的一个是：公共安全的问题
二、one of the most important questions they had to consider was （what）of public health
1）定语从句,好一一样,不再重述.
2）what=sth +that从句...这里的sth可以包括很多方面,
可以是具体的事物
如 what i need is a piece of bread=the thing that i need is a piece of bread.
还可以指代抽象的事物：
如：what i need is your love=the motion that i need is a piece of bread.
所以整句话翻译为： 他们不得不考虑的重要问题中的一个是：公共安全的各个方面.
希望能帮到您

that这里代指the most important questions

答案就是that. 等同于the question.为避免重复，才使用that.

that 可以充当表语的成分的，主 +系+ 表 结构was是系动词的嘛，后面that充当他的表语结构的