Its because "prifddinas" means "capital city". The "ddinas" (mutated form of dinas) part of the word means "city". "Prif" is used for someone or something high up, so Prifysgol or Prifathro. So yeah, technically you needed to add "city"
You’re right the dinas part is integral to the Welsh sentence. However in English both “capital city” and “capital” mean the same thing in this context and so should both be accepted.
I believe that "London is the English capital" is a equally correct translation of the actual intention of the Welsh sentence and as such, should have been accepted as correct.
I agree. It is the intention of the Welsh phrase.
Things like bwyd Ffrainc and llywodraeth Cymru are often translated with an adjective as "French food" and "the Welsh government" although they're literally "the food of France" and "the government of Wales".
In addition, you're a lot less likely to hear the phrase with an adjective in Welsh unlike in English: Llundain ydy'r brifddinas Seisnig lit. "London is the English capital", so I'd agree with your point.
"Prifddinas ydy/yw Llundain" would mean "The capital city is London" and "Mae Llundain yn brifddinas" would mean "London is a capital city". Both of them are correct in general but for the question, you would need to say the whole thing including England.
When I said "for the question", I meant the main question from Duolingo that you had to translate
"Prifddinas ydy/yw Llundain" would mean "The capital city is London"...
Probably just a typo but means “London is a capital city” as @EllisVaughan mentioned in his original comment.
No because this is an emphatic sentence. "Ydy" is the verb here and replaces "Mae".
How do you know which it is? In the similar sentence with Cardiff and the capital city of Wales, it accepted both constructions.
In constructions like these, it's a question of emphasis (shown here in italics).
You could translate:
Llundain ydy prifddinas Lloegr
as either "The capital of England is London"
or "London is the capital of England"
Prifddinas Lloegr ydy Llundain
is "London is the capital of England"
or "The capital of England is London"
In Welsh, we tend to place the emphasised word at the start of the sentence and also use our voice to emphasise it at the same time. In English (outside of Wales anyway) we tend just to use our voice. In writing, without something like italics it's unclear in English where the emphasis lies, whereas in Welsh it's obvious. That's why both English translations should be accepted here.
I went for "London is England's capital" (somewhat dictated by the words that were there I guess) and it got accepted. The word "city" was there to add but I didn't (felt it cluttered the sentence) and it was ok anyway.
I agree with Owen the translation given as the only correct answer is wrong when you actually look at the construction of the Welsh sentence. Both should be accepted.