Common Noun: Definition. A common noun is a word that refers to a general class of people, places, things, or ideas. It is not the name of a specific person, place, thing, or idea.

So if “the reason is” needs a noun to come next, that means “the reason is that” works and “the reason is because” doesn’t. Makes sense, but grammar is never so simple.

Common nouns are the names of things, that’s people, places or objects, while a proper noun is the name of a particular person, place or thing. Proper nouns always begin with a capital letter ...