"Byddwch yn dawel!"
Could tawel translate also as 'still, tranquil' (thus 'be still!' would be an alternative translation)?
Or is the shade of meaning of this 'still' only used in other contexts, e.g. with air or water (→ y Môr Tawel )?
Yes, tawel has a range of translations and 'still' is one of them.
That's been added now, although personally I'd use 'llonydd' for still.
Thank you for the clarification.
Since I learned y Môr Tawel before learning tawel as an adjective, I thought of its meaning in terms of 'calm, peaceful, tranquil', which is both still and quiet at the same time. But since Duolingo allowed only one translation, I was wondering whether bydd yn dawel! could also mean 'stay still/calm!".
The "wch" suffix is the polite form, that is, for authority figures and people you've just met. Likewise, the "a" suffix is for friends and family and the like. Neither has anything to do with plural/singular.
Um, what? Do you mean that you would use the command form bydd whether you are speaking to one friend or to three friends at once?
Because that doesn't sound like what I had read about Welsh so far.
To the best of my knowledge, Welsh is similar to French in this respect, in that tu (French) and ti is informal singular, and vous (French) and chi is informal plural as well as formal singular and formal plural.