non native english speaker? I think the "have been -ing" construction works best when it's accompanied by a phrase that indicates that you started the action in the past and it's still continuing (eg. I have been writing this..-'for a while', 'since yesterday', 'for months' 'all morning' etc.). Otherwise it sounds a bit bulky. And "I write since yesterday" or "I'm writing since yesterday" just sounds completely wrong.
I answered "I've written since yesterday."
Is it wrong because i put it in the past? If I were telling someone i had been writing something non-stop since yesterday, or if they were asking if I'd written ANYTHING since yesterday (accusing me of not working, for instance) that's how I would answer. Is that not what it's asking?