I keep confusing how to say: the room is big and the big room :( any advice on this issue?
The adjective in Arabic comes after the noun.
However, if you see the subject noun defined with AL while the adjective following it is not, then this adjective is said to be "predicative" (i.e. telling information about the noun, which is the room here). Hence, "is" is needed to complete the sentence in English.
If the adjective follows the noun in definition (as well as number and gender of course), then it is an attributive adjective, i.e. no need for "is" in English (the big room).
This is just a ... let's say a thumb rule ... to get around Duolingo.