The one collision this drill isolates (à+person → lui/leur vs. à+place/thing → y), why the drill is timed instead of untimed, a same-verb near-miss pair, the penser-à family exception this drill deliberately avoids, the replace-all vs. replace-only judgment call, and other ways to practice beyond this drill.
If the PDF doesn't display above (some browsers block embedded local PDFs), use the "Open in new tab" button instead.