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1) His neighbor described him as affable, which was actually only one of
his many qualities.
2) Her best friend said that she was rude, which was indeed her main failing.
Are the above sentences grammatical?
Are they idiomatic?
Both sentences suffer from indefinite antecedents.
While we can, without much effort, make out to whom the second "him" and
the second "her" refer, even a teeny-tiny unnecessary effort is something
of a speed bump in communication.
Maybe better as:
1a) His neighbor described him as affable, but affability was only one of
his many [fine/pleasant/likable/&c] qualities.
2a) Her best friend said that she was rude, and rudeness was indeed her
Not polished gems of prose, but they make clearer who has which qualities
with only minimal rephrasing.