How do you use opine in a sentence?

"Opine" is an intransitive verb, and in all cases it can replace the phrase "give the opinion". It doesn't have a direct object. You can't opine someone, but you can opine aboutsomeone.

You might write an article about the virtues of the macroeconomic theory of Keynes, to which I would opine that Keynes was an idiot and that Friedman has already refuted him.