"Surat ini belum ditandatangani."

Translation: This letter is not signed yet.

[BrianG21]

Why is it always yet. This letter is not signed seems acceptable.

[WayangOrang]

belum = not yet
tidak = not

Surat ini belum ditandatangani = This letter is not signed yet.
Surat ini tidak ditandatangani = This letter is not signed.

[JimMacFly]

Shouldn't "not yet signed" be accepted as well?