Rebuild the missing context
Assume the recipient will not remember every date, promise, or earlier conversation. For a message about how to apologize for leaving trash in a shared area, use this factual base: state the date, place, property, noise, pet, or shared-space impact without arguing about the neighbor's character.
Say what needs to happen next
State the decision, information, or practical action you need and include a real deadline only when one exists. 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.
Avoid making urgency into pressure
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. Do not make the other person responsible for reducing the discomfort that prompted you to write.