Could you say "Tawelwch" or is that only used as a noun and not as a command?
Tawelwch is just a noun. "Taw" might be an acceptable alternate answer as would "Byddwch yn ddistaw" as would the "Bydda" version of "Byddwch"
Bydda'n dawel ?
Why not "Bod yn dawel!"? Bod is included in the drop-down menu, but not accepted in my answer.
The drop-down hints are not context sensitive. The command form of bod is needed here: bydd!/byddwch!. See the course notes for examples and explanations.