my techer says

[olaf]

one of them is not perfectly correct, I don't see which one, do you?
There is a dirty mark on the rug which must be removed.
On the rug, there is a dirty mark which must be removed.
thanks for help

 

[naomimalan]

For me, both are perfectly correct. However, the first one could also mean that it is the rug which must be removed, not the dirty mark. Nevertheless, common sense would lead us to reject this interpretation and choose the more obvious interpretation (it's the stain that must be removed.)

If you place a comma (,) before the "which" in the first statement, then there can be only one possible meaning: it's the stain that has to be removed.