日語 Japanese Ask Questions Thread2 質問

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Japanese - Ask questions thread
http://dis.4chan.org/read/lang/1174719097/l50

Question about counting:

I was taught to say something like
物が一つある
but I have seen things like
一つの物がある

My instructor tells me to never use the latter, is it straight up wrong/improper/impolite/my instructor is retarted?