I am sure mrs. O will appreciate a letter from you. Sorry about Mr. O's passing.

In Denmark, we said "De" to people we didn't know and also to older people, and "du" to friends and family. In the sixties sometime, it became "in" to say "du" to everyone, but especially older people had a hard time adjusting - no wonder! I still tend to say "De" to people around 80 or more - it feels respectless not to.

Both words mean "You".