have a good command of

have a good command of something

to know something well. Bill has a good command of French. Jane has a good command of economic theory.
See also: command, good, have, of

have a good command of

Have the ability to use or control; have mastery of. For example, She has a remarkably good command of Japanese, or He had a good command of his emotions. [Mid-1600s]
See also: command, good, have, of