have a good command of

have a good command of (something)

To be knowledgeable about or skilled in a particular area or thing. If you don't have a good command of the course material, then you won't do well on the exam.
See also: command, good, have, of
Farlex Dictionary of Idioms.

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
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

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
The American Heritage® Dictionary of Idioms by Christine Ammer.
See also:
  • have a good command of (something)
  • know (one's) onions
  • know your onions
  • know your stuff
  • know one's stuff
  • know stuff
  • know (one's) stuff
  • know about
  • know about (someone or something)
  • triple threat