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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.