You look as if you saw a ghost.

[GeneD]

You look as if you’ve seen a ghost.
I feel as if I’ve run a marathon.
The above examples are from Raymond Murphy's "English Grammar in Use". What confuses me is that in the explanation part he mentioned unreal examples and the unreal part was in the past (I don't like Tom. He talks as if he knew everything.). In the first two sentences, the meaning is also unreal, isn't it? They didn't see a ghost and run a marathon.

Is it necessary to use the past in unreal sentences? Is He talks as if he knows everything possible? What about You look as if you saw a ghost?

 

[jutfrank]

Is He talks as if he knows everything possible?

Yes, that's fine.

What about You look as if you saw a ghost?

have seen is more appropriate and natural. At least in British English.

 

[GoesStation]

You look as if you’ve seen a ghost.

Yes, that's fine.

have seen is more appropriate and natural. At least in British English.

Illogically but idiomatically, we Americans usually say You look like you'd seen a ghost. The past perfect really doesn't belong there, but that's what we use in this fixed phrase.