¿Cuál es la diferencia entre common y commonplace ?Puedes dar oraciones como ejemplo.

Commonplace can also be used to say that something is "ordinary", "mediocre","unremarkable" or "not unusual", with it being a normal occurrence, nothing out of the ordinary e.g. "Having a smartphone has now become commonplace amongst school kids"

While "common" can be used where there are shared characteristics. e.g. "My friend and I have a lot of things in common"
"American and British English have a lot in common"
You cannot use "commonplace" in this context.

“This phrase is so ( ) that you would see it in text books of the standard Japanese.”= You can use either common or commonplace, but commonplace sounds more formal and is used mostly in books, news, articles etc. In casual conversation, "common" is used more.

"This phrase is so common that you would see it in standard Japanese textbooks" sounds more natural.