"I want a white dress."

Translation: Dw i eisiau ffrog wyn.

[molly-petrichor]

Also, ffrog is a feminine noun, and gwen is the feminine form of the adjective?

[loopylinda]

Why is gwyn wrong?

[AnUnicorn]

Adjectives soft-mutate after feminine nouns, and g soft-mutates by dropping.

[Susan783746]

FYI, I had the exact wording as the answer but it was counted wrong.

[ibisc]

Unless you say what kind of question it was and exactly what your complete answer was, there is no way to tell what the problem might have been.