This question already has an answer here:

These statements are ok:

  1. "I looked George up."
  2. "I looked up George."
  3. "I looked him up."

But this is not ok: "I looked up him."

In this case, it's not possible to substitute the pronoun "him" for "George". What rule prohibits this substitution?

marked as duplicate by snailboat Feb 4 '17 at 4:26

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  • The final constituent has to bear stress, and personal pronouns normally don't bear stress, so they generally don't appear in this position. – snailboat Feb 4 '17 at 4:27
  • In other words, the order 'verb + particle + object' is not possible if the object is an unstressed personal pronoun. So we can replace "George" by unstressed "him" in "I looked him up" but not in "I looked up him". – BillJ Feb 4 '17 at 10:04