Inspired by this answer and the comment by @jgn, it made me dig deeper into the wording of the Geas spell, and how it doesn't seem to tell you if you can only give commands at the beginning, delay the command for another time, or even continue giving additional/new commands. The beginning sentence seems to hint at a single command (emphasis mine on the singular nature of the wording):
You place a magical command on a creature that you can see within range, forcing it to carry out some service or refrain from some action or course of activity as you decide.
And also later:
You can issue any command you choose…
But in the middle when explaining what happens when it ignores you:
…it takes 5d10 psychic damage each time it acts in a manner directly counter to your instructions…
which could have easily been made singular if it was a single command.
this answer to a different Geas question seems to say that it's a singular command, but it's not backed up by anything, and doesn't say when you can issue the command.
So my question is: When do you / can you decide what command(s) the affected creature should follow?
Best Answer
No, Geas is a single command
Geas (PHB, 244) uses language based on the singular when discussing the command (emphasis mine):
That single command can be to
But what about "instructions"?
The explanation of what service you want or course of action/activity may involve instructions so it's clear what you want, but it's still the single command and single service/action/activity.
As long as the instructions are part of carrying out the single task and not separate tasks in and of themselves. The spell is very clearly about issuing a single command and information needed for it to be successfully executed or for the punishment for non-obeisance to be clear. '
A single command. Once.
You give your command and any associated instructions to follow it through as you cast the spell, and then it's over. There are no further commands, just a penalty for not following the command.
You can end the spell early, but you can't make changes, addendums, or new commands without recasting the spell.
DM Rulings
As always, a DM is free to interpret your command request in whatever way they think is reasonable. If your instruction string makes sense to them, they can allow it. If it doesn't, they can opt not to and explain why. Generally, I think most DMs would be reasonable in assessing if the instructions seem like trying to game the system, like using wish to get more wishes, or if it's a logical and legitimate instruction string for the command.