"She read the book."
Translation:Darllenodd hi'r llyfr.
May I not push the verb and the definite article together 'ddarllen'r llyfr' ?
Not sure exactly what you mean.
If you wanted to say just 'reading the book' it would be darllen y llyfr - no 'r possible there. See the explanation in the notes for the section 'The' about how to decide when to use 'r/yr/y.