"What should she do tonight?"

Translation: Beth ddylai hi wneud heno?

[AbunPang]

Can you not insert an "ei" here: "beth ddylai hi ei wneud heno"?

[ibisc]

Yes, if you wish. It is usually left out in the colloquial language, although the mutation remains.

[AbunPang]

Thanks, I'll test it again next time then.