This building is old = هذه البناية قديمة
This is an old building = هذه بناية قديمة
The removal or addition of (AL) to بناية makes the difference.
Speaking of English first:
This building is old: Here, the subject of the sentence is This building altogether, because it is the term that comes BEFORE "is". On the other hand, the adjective old is after "is" so it serves as "predicative". Predicative in the sentence is the information we want to tell or say about something. The end status of the thing (the subject).
This is an old building: Here, the subject (which is before "is") is only the term This, while the predicative or the information we want tell about This thing is an old building (again, because it comes after "is"). The adjective old here is not alone, but it is attached to building and they together form a predicative of the sentence.
In the examples above, the adjective old in the first sentence is said to be predicative adjective because it tells the status or the ending result of the subject.
In the second example, the adjective old is said to be attributive adjective because it attaches itself to the noun and becomes like part of it, and together they act as one unit.
If you are familiar with Turkish, this is like the difference between Ev eski (the house is old) and Eski ev (the old house), or maybe in Russian as (дом - старый) for (the house is old) and (старый дом) for (the old house).
In Arabic, same concept applies but the switch here is in adding and removing (AL). Mainly, because nominal sentences in Arabic (sentences that begin with nouns and not verbs, and can be completed without any verb sometimes), must have their subject defined with (AL). So, هذه البناية is automatically assumed to be a subject (the thing before "is") and the adjective قديمة is the predicative alone (the predicative in such sentences is always indefinite).
Thus, if I remove (AL) from بناية to make it indefinite, this is like we are automatically pushing it forward to make it a predicative of the sentence (the thing that comes after "is") along with the adjective قديمة which now comes an attributive adjective and not a predicative adjective (because it is not alone, it describes building on its own now).
Hope this clarifies things.
Why is it that when I listen to the whole phrase I hear sounds that do not come up in the "word box" audio? I would understand that the world boxes have the full phonetic sequence of a word and that in the sentence some sounds are suppressed due to syncopation but the other way round? help !!! U_U