>>118
Both sentences are grammatically correct, but their meanings are slightingly different. The difference are so subtle that literal translation into English often loses the nuance.
I think your instructor taught you an err-on-the-right-side strategy. The former sentence structure is neutral and virtually always fits in. You can specify the implication to some extent by using the latter structure. But it may sound odd if the specified implication doesn't match the context.
I can't verbalize the nuance so I just give examples:
If you say 物が一つある, then the listener may say "then?" But if 一つの物がある is used, the lister may feel "like what?"
So, when you start a conversation by 窓が一つある (I see a window), it can go in any direction. But if you say 一つの窓がある, then the following sentence most likely describe the window.
tl;dr: Use 物が一つある and you'll be safe.