More and more people have realized that how serious this problem is.

问题描述:

More and more people have realized that how serious this problem is.
这个句子中that为什么要删掉?

More and more people(主语) have realized (谓语)how serious this problem is(宾语从句).
从句的引导词是关系副词how,已经有了.所以不能再有引导词that了.