have a good command of

If someone told you they have a good command of English, would you understand them? To have a good command of something means to know something very well. So I could say for example that I have a good command of History if that was really the case. You could say you have a good command of economic theory and so on and so forth. That's a very idiomatic way of saying you have all the knowledge related to some specific subject. So tell me, now that you know that expression, do you think you have a good command of English? Let me know in the comment section. Talk to you next time!