が/の conversion (modifying clause subject)

Inside a noun-modifying (relative) clause, the subject marker が often alternates with の — 私が書いた本 / 私の書いた本 ('the book I wrote') are both grammatical and equivalent. Common in writing and considered slightly more elegant or compact when の is chosen; learners should recognize both. The substitution applies only inside modifying clauses, not in main clauses.