Grammar question

Sorry, I couldn't think of a suitable existing discussion to lodge this under. I was moaning (a little dramatically) last night about being a terrible artist in an IM conversation, and said, "我不好画家", but I was corrected to "我是个不好的画家" to make the sentence smoother.

I'm a bit confused as to when we miss out the joining words like 是 and 的 and when we don't. Is it just down to experience, knowing which sentences require them?