"هٰذِهِ الْبِناية واسِعة."
Translation:This building is spacious.
This also used to confuse me, but I now realized that ال is also used as a "definite article" that is when you know the object you're talking about for example, This kitchen, this dog, this building, etc. But when ال is absent, the object could not be specific. That is, a kitchen, a dog, a building, etc. Which could be any kitchen, any dog, any building etc.. I hope this was made sense and was helpful
In MSA, you need to use the definite article with demonstrative adjectives, so you say "this the building," rather than "this building." To say "the spacious building," "spacious" would need a definite article of its own. And we'd normally use a pronoun as a "copula" of sorts to make the sentence less ambiguous:
هذه هي البناية الـواسعة
"This is a spacious building" means you still haven't established that it is a building to begin with. Maybe it is so spacious your friend thinks you're outside, so you say that "this," which we still don't know is a building, "is a spacious building," so spacious that its status as a building was not apparent. In Arabic, you'd express that without any definite articles, so that هذه is all alone in the subject position, since you'd need an article to connect a noun to it:
- هذه بناية واسعة.
But "This building is spacious" means both the speaker and the interlocutor know that what the former is referring to is a building. It is the fact that it is spacious which is new information. In Arabic, you'd use a definite article for that:
- هذه الـبناية واسعة.
This exercise uses the latter.