Recognize what makes this moment difficult
Acknowledge the event directly without turning it into a lesson, comparison, prediction, or silver lining. For a message about how to apologize for taking a package by mistake, use this factual base: state the date, place, property, noise, pet, or shared-space impact without arguing about the neighbor's character.
Offer support that can be accepted
Offer one specific form of practical or emotional support and make clear that no immediate reply is needed. Offer cleanup, replacement, access, a schedule, or another practical remedy you can complete. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Follow their pace rather than yours
Do not threaten, shame the neighbor publicly, or involve the whole building in a private issue. Use property management, local authorities, or emergency services for genuine danger rather than confronting someone unsafely. The recipient decides whether they want conversation, ordinary company, practical help, professional support, or quiet.