An American who is from Chicago and who is learning Chinese at a nearby college, might do well to have a go at learning English first. A conversation between him and one of my British colleagues went something like this.

Chicagoan: I wish people would talk more in class.
Brit: They are too reticent?
Chicagoan: Reticent? What's that? It's not a real word.
Brit: Of course it is. It means shy or diffident.
Chicagoan: Diffident? You made that up too. They aren't real words.

And he wasn't joking!

Oh, dear. My son thinks the Brit made up the story, but I believe you.

I was there. I've just simplified a rather longer conversation.

We have pointed him to Webster and to the AHD.

It's embarrassing.