To extend the previous response: - I think a "dent on" is OK, but I personally would say a "dent in". - Maybe the word you are reaching for is the "lid" of the laptop.

Are the prepositions correct in these cases ? the laptop has a dent on the left side CORRECT The laptop has a USB slot at the back CORRECT the book has a summary on the back cover CORRECT the book has interesting pages at the back CORRECT / in the back??? INCORRECT Can we call it : the front of the laptop / the cover of the laptop ?? To me it's unclear which part of the laptop you are referring to. A laptop 'cover' is normally a protective skin or case to keep your laptop in. The 'front' of the laptop is also unclear to me: it could refer to the screen, which is at the front when the laptop is open. Or it could refer to the area nearest to your wrists when you are typing.