Having mastery of a subject is to have great knowledge. It is also the state of being in command or control.
Ex: Yo Yo Ma's mastery of the cello made him a fixture in the music world. His talent made him an obvious choice to play at the last presidential inauguration.
Ex: I finally stopped doing what everyone else thinks is best for me. Now that I am making my own decisions, I feel as though I have developed mastery of my own life.
A master is one who is superior in some field of knowledge or the arts. If you take a master class in ballet, for example, the teacher will be a master of that dance form. A jack of all trades is a master of none describes someone who can do a little bit of everything but is not really good at any of them.
Ex: Some of the electrical outlets in our house didn't work, so we hired a master electrician to put in new wiring. Now every outlet works all the time, and we also feel safer in our home.
Ex: My father is just a jack of all trades but a master of none. In fact, whenever he repairs something in the house, we have to hire an expert to fix it again.
To command is to be in control of an event or a group of people, or to give orders. A command is an order given by someone in authority. Command is often associated with the military, especially the navy, where a commander is an officer.
Ex: Generals are officers of the highest rank who have the responsibility of commanding the troops in battle. These leaders train for years to know the most successful tactics to use.
Ex: Failure to follow the commands of safety officers during a building evacuation can lead to individuals being injured or trampled.
To be in command is to be able to manage other people, situations, or your own emotions. To have a good command of the English language is speak it and write it correctly. Your wish is my command is an idiom that means I will do whatever you want, and it usually said to someone who is asking for too many things.
Ex: Jacob was securely in command of the wrestling match when a movement in the spectator section distracted him. In no time, his opponent had pinned him, and Jacob lost the match.
Ex: My little sister is such a princess! Every time she asks for something, she insists that I say, "Your wish is my command".