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.

If you choose the third mode and put a land card onto the battlefield, that doesn't count as playing a land. For example, you can both play a land and put a land onto the battlefield this way during your turn.

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.)

The first mode won't affect life that was gained earlier in the turn.

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.

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.

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.