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" This book is a favourite of mine. " is grammatical. How about the sentences below?

  • He is a friend of Tom? He is a friend of Tom's?

  • He is a friend of my brother? He is a friend of my brother's?

  • He is a friend of my manager? He is a friend of my manager's?
Thanks a lot!
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Haohaoxuexi" This book is a favourite of mine. " is grammatical. How about the sentences below?
  • He is a friend of Tom? I would not use this one.
  • He is a friend of Tom's? Typical wording
  • He is a friend of my brother? I would not use this one.
  • He is a friend of my brother's? Typical wording
  • He is a friend of my manager? I would not use this one.
  • He is a friend of my manager's? A bit awkward, but more typical than your previous sentence in my opinion. I might say "He is one of my manager's friends."
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How about these sentences:

  • This is my extension. This is the extension of mine.

  • This is Tom's extension. This is the extension of Tom's.

  • This is my favourite. This is a favourite of mine.

  • This is my manager's favourite. This is a favourite of my manager's.
Thanks
HaohaoxuexiHow about these sentences:
  • This is my extension. OK
  • This is the extension of mine. No
  • This is Tom's extension. OK
  • This is the extension of Tom's. No
  • This is my favourite. OK
  • This is a favourite of mine. OK
  • This is my manager's favourite. OK
  • This is a favourite of my manager's. OK
Where I wrote 'No', it might be possible to use that wording in a longer sentence and/or a very limited context. For example:
This is the extention of Tom's that currently works. None of his other extentions are functional at the moment.