Now I am confused - so far everything that has been taught about "is" as a positive statement has been using "mae" in the third person, e.g. "Mae Megan yn canu" etc. We have been told that "ydy" is used when asking a question in the third person, e.g. "Ydy hi'n gweithio?". So how come ydy suddenly appears in this expression to mean "is"? Does that mean I could have been saying all along "Megan ydy x", rather than "Mae Megan x" to mean "Megan is something or other"? Why isn't it "Mae Cymru yn braf"?
Because in sentences where you are identifying the profession, name, identity, etc. of the subject, you put that bit before the verb, and that changes the verb to the question form. ''Dw i'n lwcus'' ''Cath lwcus dw i.'' ''Mae hi'n lwcus'' ''Cath lwcus ydy/yw hi''
Sorry, could you explain this with different examples? You're saying "I am lucky" and then "I am a lucky cat", is there any reason for this? I can kind of get it from your earlier explanation in the post and thinking back of "Fred dw i", but then the examples really threw me. Diolch!
The second two sentence examples are just third person singular equivalents of the first two sentences, the final one corresponding with "Gwlad braf ydy Cymru". Sorry I can't explain grammar definitions though.