"هٰذِهِ الْغُرْفة جَميلة."
Translation:This room is pretty.
I made the mistake of writing "this pretty room" but because it's hadha al-ghurfa jamilah, the "al" doesn't match Jamila, which is the hint.
From my experience with other languages here, the hint is just a guide and not exactly the solution. It got me in troubles many times as well. The hint seems to show the word in definitive case, and leaves you the rest to figure it out.
yeah sorry it's late, I forgot the name of the grammar rule that explains why it works the way it does when the noun has the article but the verb does not.
Well, the verb never gets an article (does it in English?).
Anyway, as a rough guide, here are two sentences below and their Arabic counter part:
1. This is a beautiful room: هذه غرفة جميلة.
2. This room is beautiful: هذه الغرفة جميلة.
One can say also (This is the beautiful room), and it will sound as it is emphatic in Arabic: هذه هي الغرفة الجميلة.
I can go deeper into grammar if you want to but maybe as a beginner you wouldn't want that and rather focus on the vocabulary :)