1. He lent me a book, which interested me very much.

2. He lent me a book, and it interested me very much.

3. He lent me a book, interesting me very much
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Can #2 be changed into #3? Are they all grammatical and the same in meaning?

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4. I trust him, who is very honest.
5. I trust him, for he is very honest.
6. I trust him, being very honest.
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What about these ones? Are they all grammatical and the same in meaing? Is #6 correct?

The -ing phrasing is not the best. It's not incorrect; it's just not used very much.

1, 2, and 5 are the best sentences here.

No. 1 no comma required

No. 2 correct
No. 3 wrong

In the first set of sentences:

1. "He lent me a book, which interested me very much."
2. "He lent me a book, and it interested me very much."
3. "He lent me a book, interesting me very much."

Out of these three sentences, #2 is the most common and natural-sounding option. The conjunction "and" connects the two clauses, indicating that the action of lending the book took place, and it interested the speaker very much.

Sentence #1 is also grammatically correct. The relative pronoun "which" introduces the relative clause "which interested me very much." It describes the book that was lent to the speaker as the one that interested them.

Sentence #3, while grammatically correct, sounds less common and may sound slightly awkward. The sentence structure is different from the other two, as the main clause ("He lent me a book") is followed by a participial phrase ("interesting me very much"). Although it is grammatically acceptable, it may not be as commonly used as the other two options.

Regarding the second set of sentences:

4. "I trust him, who is very honest."
5. "I trust him, for he is very honest."
6. "I trust him, being very honest."

Sentences #4 and #5 are both grammatically correct and convey the same meaning. They express trust in someone based on their honesty. Sentence #4 uses the relative pronoun "who" to introduce the relative clause "who is very honest," emphasizing the honesty of the person being trusted. Sentence #5 uses the conjunction "for" to introduce a reason or explanation for the trust: "I trust him, for he is very honest."

Sentence #6, "I trust him, being very honest," is grammatically correct but sounds less natural in this context. While using a present participle phrase can be grammatically acceptable, it could be ambiguous or confusing since it may imply that the speaker is "being very honest" rather than the person being trusted.

Overall, sentences #4 and #5 are both commonly used and convey the intended meaning, while sentence #6 may be less common and could be confusing to some readers.