For example, take the spell command:
You speak a one-word command to a creature you can see within range. The target must succeed on a Wisdom saving throw or follow the command on its next turn. The spell has no effect if the target is undead, if it doesn't understand your language, or if your command is directly harmful to it.
As the spell takes effect next turn, the results of the spell are not immediately obvious. Is the DM required to announce whether the creature has succeeded or failed on its saving roll? Or, in the case of command, whether the command was appropriate or whether the creature understood the command?