English grammar

I don't think anybody in British English countries says "This is a pen of my father". I suspect they don't in America either.

I would say: "This is my father's pen."

Or: "This pen belongs to my father."
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I prefer an economy of words.

Centrox is correct--neither of those expressions would be American usage. One might say: "This is my father's pen." or "This pen is my father's." Another problem you have is the use of the indefinite article, "a." The implication is that your father has more than one pen. In such a circumstance, a native speaker would be likely to say: "This is one of my father's pens."

Thanks very much, Setanta. I would also say "This is my father's pen." I took this example from a Longman English Grammar Practice book, where it says that instead of saying "This is my father's pen" you can say "This is a pen of my father's" - NOT "This is a pen of my father." There could be lots of similar examples, by is "of my father's" the only grammatically correct way of saying it. I have never heard anybody in the U.S. say that. So, would "of my father" be grammatically wrong in AE?

Thanks very much, Centrox. I would also say "This is my father's pen." I took this example from a Longman English Grammar Practice book, where it says that instead of saying "This is my father's pen" you can say "This is a pen of my father's" - NOT "This is a pen of my father." So, basically, my question is whether "This is a pen of my father" is grammatically incorrect or not, and is there any difference between BE and AE with regard to this? Thanks in advance!

"This is a pen of my father" is grammatically correct in a formal sense, it just isn't used, either in US or British English.

Thanx!