[quote="Lenka"]I have seen this sentence: (btw: is this sentence I have just written written correct and right? I still have big problem with using present perfect :) )
Your present perfect seems just fine.
In spite of the fact that this 's' in "John's" seems to be unnecessary there, it is very widely used. It is called 'double possessive'. In your example, the meaning would be the same without an 's', but in other cases it is very helpful. For example:So here is the sentence: ....a friend of Sir John's.
Can you tell me why there is " 's " at the end of John ?
D we have to use " 's " always? I mean for exmple: This is a table of my grandmother's. Is it right?
"A picture of my sister"....here the meaning is ambiguous. You do not know whether it is your sister that is seen in a picture, or it is a picture belonging to your sister. That is why it is better to say, 'A picture of my sister's' so you know that it is your sister's picture.
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