The English for this sentence is “He told us to keep quiet.”
Should the French be “Il nous a dit de rester calmes”? This reflects the past tense of the English and puts the adjective in the plural to accord with “nous”.
Also, is “tranquilles” okay instead of “calmes”?
Your guess is correct for the tense and the plural.
Concerning the “keep quiet”, neither “rester calme” nor “rester tranquille” work best here.
Because “keep quiet” is about not saying anything, while “rester calme/tranquille” is about not being emotionally invested (about movement, speech, etc). Actually it is the direct translation of “to keep calm”.
So here the best translation would be IMHO “Il nous a dit de nous taire.”
Se taire is exclusively about not talking at all.
Thank you.
To go a little further, does “se taire” have to do only with talking? I ask because although “keep quiet” would probably be interpreted as “keep from talking”, more broadly it could also mean “keep from making any noise through talking or movement”, and could even include, I suppose, any devices being carried, e.g. if the people involved were trying to be sneaky.
Perhaps “rester silencieux” is also a decent option?
Se taire is only about not talking.
You can also use “rester silencieux”, but you can also use “ne pas faire de bruit” (=don’t talk and don’t make any noise).
“Il nous a dit de ne pas faire de bruit.”.
However, you lose the information of being quiet at first (found in the use of “to KEEP quiet”, “RESTER calme/tranquille/silencieux”).
Then, in most case when you ask people to keep quiet, you usually don’t care that they were quiet to begin with, as you are mostly interested in them being quiet from now on.
All those sentences work, there is only a subtle difference between each. And the context will tell you which one fits best as usual with contextless translations…