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(1A): John bought a book for Mary, but then decided to keep it.

(1B): John bought a book for Mary, but Mary lost it.

(2A): John bought Mary a book, but Mary lost it.

(2B): John bought Mary a book, but then decided to keep it.

I think (1A) and (2A) sound more natural than (1B) and (2B) respectively. If so, then there must be a subtle difference between (3A)“John bought a book for Mary” and (4A)“John bought Mary a book” in terms of meaning. (4A) has the implication that the book had already been transferred to Mary, while (3A) doesn’t.

Am I right?

Thank you!
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Liveinsea(3A)“John bought a book for Mary” and (4A)“John bought Mary a book” ... (4A) has the implication that the book had already been transferred to Mary, while (3A) doesn’t.
Setting aside the alternate meaning mentioned by Mr. M. (that 3A can mean that John bought a book because Mary requested him to buy it), 3A and 4A are the same in terms of whether the book has already been transferred to Mary. There is no strict logical implication that if you say "John bought Mary a book", Mary already has the book, because the following is not at all contradictory: John bought Mary a book, and he's going to give it to her for her birthday next week.

CJ
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Hello, Liveinsea,

as the other contributors have pointed out, the difference in meaning is non-existent, at least this can be resumed after a substitution test:

John bought Mary a book/John bought a book for Mary, but hasn't presented it to her yet.

However, there are indeed grounds for a certain extent of ambiguity considering the fact that for can mean either in place of sb or to the benefit of sb, but the situational context will serve as a rock-ribbed practical indication of which meaning to accept.

Leaving aside these matters, let me comment on the structural issues. The verb bought is monotransitive (taking only a direct object) in John bought [a book - direct object] for Mary, but ditransitive (with two objects) in John bought [Mary - indirect object] [a book - direct object]. Semantically, John is an agent, a book - an affected participant, and Mary - a recipient (or, as some linguists say, a beneficiary). There is a rule saying that an animate indirect object (as a beneficiary) is completely synonymous with a for-phrase. Thus:

John bought Mary a book. = John bought a book for Mary.

Respectfully, Gleb Chebrikoff
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Comments  
There is no 3a or 4a, but I suppose I understand your intention.

There is no such difference in meaning. The only possible ambiguity (in #1a, b) is that John bought the book on Mary's behalf, i.e. that Mary was originally to buy the book. #2a, b sound more 'natural' to me.
 CalifJim's reply was promoted to an answer.
Hi

There's really very little difference, but I'd use 2A and 2b because the first clause in each is more compact.

Forget any implication you may perceive about the book having been already transferred to Mary. There is no semantic indication whatsoever of that.

Best

Bill J
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 Andriy Lapin's reply was promoted to an answer.
Thank you, all of you! You are so nice and kind!