In my textbook I learned that when a sentence shall be nominalized, there are these 2 ways.
SENTENCE + こと
SENTENCE + という + こと
2 is in case a complex sentence shall be nominalized.
In addition, when the to be nominalized sentenced ends with a な adjective, it shall be nominalized this way:
な adjective + なこと or な adjective + である
Now, concerning this case, there were 2 examples in the grammar section, one for simple sentence and one for a complex sentence.
世界中でこの漫画が有名な (な/である) こと知っていますか。
日本に来てから、家族はとても大切 (だ/である) ということに初めて気がついた。
Now, for the complex sentence, would it be possible to write ...家族はとても大切なということ... as well?
My textbook gave me no explanation as to why it only proposes the copula, but not the な suffix DESPITE claiming otherwise just a few lines before :/ Or is it possible that in the nominalization of complex sentences requiring という, it is obligatory to use the copula when the to be nominalized sentence ends with a な adjective?