My initial translation for "Het is niet te doen" was " There's no way that can be done", or, "It can't be done". Does anyone else understand the meaning that way? I would have thought that the solution "It musn't be done" would be translated as "Het moet niet gedaan worden".
My initial translation would be 'it is not doable'.
'There's no way that can be done' would be 'Er is geen manier waarop dat gedaan kan worden'.
'It can't be done' would be 'Het kan niet gedaan worden'.
'It is not to be done' sounds to me like 'It should not be done', which would be 'Het zou niet gedaan moeten worden'.
It does actually. Perhaps this is one of those expressions you just have to remember. If you say "Het is te doen" it means something can be done or is possible to do. By adding "niet" you're saying it cannot be done, or is impossible to do. See also vandesnieder's comment above. And Susande gives "unbearable" as an alternative meaning of this expression. Make sense?