Silumgar's Command

• WWizards of the Coast2015-02-25
  Official Ruling
  You choose the two modes as you cast the spell. You must choose two different modes. Once modes are chosen, they can’t be changed.
• WWizards of the Coast2015-02-25
  Official Ruling
  You can choose a mode only if you can choose legal targets for that mode. Ignore the targeting requirements for modes that aren’t chosen. For example, you can cast Ojutai’s Command without targeting a creature spell provided you don’t choose the third mode.
• WWizards of the Coast2015-02-25
  Official Ruling
  As the spell resolves, follow the instructions of the modes you chose in the order they are printed on the card. For example, if you chose the second and fourth modes of Ojutai’s Command, you would gain 4 life and then draw a card. (The order won’t matter in most cases.)
• WWizards of the Coast2015-02-25
  Official Ruling
  If a Command is copied, the effect that creates the copy will usually allow you to choose new targets for the copy, but you can’t choose new modes.
• WWizards of the Coast2015-02-25
  Official Ruling
  If all targets for the chosen modes become illegal before the Command resolves, the spell won’t resolve and none of its effects will happen. If at least one target is still legal, the spell will resolve but will have no effect on any illegal targets.

Rulings via Scryfall