First, here's a material that made me keep thinking about this issue in the first place.
I felt as if I had been hit by a hammer.
I wasn't hit by a hammer, but I had a headache like a person who was hit by a hammer.
I felt as if I was hit by a hammer.
I was hit by something, but I'm not sure what exactly hit my head. Seeing as there was a hammer when I first walked in, I guess it was a hammer that hit my head.
Am I right to think this way? I'm really want to get used to using 'as if' naturally.