Questions & Answers about Мой друг не осуждает людей за ошибки.
Мой друг is in the nominative case, because it is the subject of the sentence – the one doing the action of not judging.
- друг (friend) is a masculine noun.
- The possessive мой agrees with the noun in gender, number, and case.
Possessive forms in the nominative singular are:
- мой – masculine (мой друг – my (male) friend)
- моя – feminine (моя подруга – my (female) friend)
- моё – neuter (моё письмо – my letter)
- мои – plural (мои друзья – my friends)
So you use мой here because друг is masculine singular and is the subject.