"My bedroom is spacious."
Translation:غُرْفة نَوْمي واسِعة.
Not sure if there is a website for non-Arab learners. Anyway, what I think Duolingo is doing is that they are teaching a traveler's Arabic, where grammar is not critical. It's a mix between standard and dialectical (sometimes from the Levant dialects and some other times from Egyptian).
Ta-Marbútah is a combination of (H) and (T). When the word is isolated or simply not to be followed by something else, then it is usually spelled (H). However, when there is something afterward, and the letter must bear some vowel, then it is flipped to (T).
Harakat at the end of the words are "some" of the signs of declension of the word. These signs change accordingly if the word is dual or plural (and there are other special categories).
However, to simplify the matter down to the basics:
- Nominative: In this case, the simple word ends with (-u); If undefined with (AL) then it becomes with Tanwin (-un).
- Accusative: In this case, when the word receives the action of the verb, the word ends with (-a); When indefinite, it becomes Tanwin (-an).
- Prepositional: When the word is preceded with a preposition, its end receive (-i); When indefinite, it becomes (-in) with Tanwin.
- Genitive: When two words follow each other, or what is typically called (OF-relation), then the second word is treated like the prepositional case, with (-i) and (-in).
This is just a brief. Things would change if the nouns are plural or dual and so on.