"He can't sing."
Translation:Dyw e ddim yn gallu canu.
Dydy e ddim yn gallu canu - 'He cannot sing' (he sounds like a crow in a bucket)
Dydy e ddim yn gallu ei ganu - 'He cannot sing it' (a difficult song, perhaps)
'He cannot do...' Well, cannot do what? There needs to be something there that he cannot do, so...
- Dydy e ddim yn gallu (ei) wneud - He cannot do (it). The ghostly ei, even if it is not written down or actually spoken, still causes the mutation of gwneud -> wneud In more formal Welsh the ei would actually be written down and perhaps said out loud.
Where a mutation is not 'obvious' it is often because of some ghostly particle like that. It all adds to the fun...