事(こと)なの(A) is (Description of A), (A) means that (Description of A)

When the noun こと (or 事(こと) in its kanji form) is combined with なの, it conveys an explanatory meaning that is similar to what can be seen in the んです・のです grammar pattern. As with んです, ことなの transforms 'it is (A)', into 'it is that it is (A)', simply emphasizing the point.

ことなの may be seen in several different forms, and is quite often used in conjunction with というのは. In these types of sentences, というのは will present the topic, before ことなの will be used to show the conclusion that the speaker has drawn about that topic.

This expression may be used with any word in its attributive form, nouns followed by の, or な-Adjectives followed by な.

という may also be used directly before ことなの. However, this is not always the case, and is even more unlikely when it was already utilized in the first half of the sentence.

Caution

As というのは is quite long, it will often be shortened to って or とは in casual speech.

Caution

ことなの is also frequently used when asking questions. This is usually when the speaker has drawn a conclusion about something that has been said, and wants to confirm their understanding.

なのか is very frequently heard/seen in drama, novels, manga, and similar types of media, but may be considered rude in daily speech.