The notes people avoid writing are the ones that matter most — sympathy above all. Today: the three hardest occasions, disarmed.
Sympathy: write plainly, write anyway. You are not there to shrink the sadness; you're there so they're less alone inside it. "I was so sorry to hear about your mother's death" — the plain word, plainly said, is respect. If you knew the person, one specific memory is worth more than any composed comfort. Strike "everything happens for a reason," anything starting with "at least," and replace "let me know if you need anything" with an offer that has a date on it. (The full guide — bookmark it before you need it.)
Congratulations: be a witness. Skip the generic praise; testify to what you saw — the late nights, the second attempt, the nerve it took. "I watched you build this" outranks "you're amazing" every time.
Thinking-of-you: the highest form of mail. No occasion needed — the trigger that brought them to mind IS the note, three sentences, with the magic clause: "no reply needed." Unprompted regard is the most valuable thing paper carries.
Your assignment
Somewhere on your conscience is the overdue card — the condolence you never sent, the congratulations that got away. Its statute of limitations never runs out. Set the tool to the occasion, borrow the opener, five sentences, post it. This one's the reason the course exists.