The owner of printing depot personally wrote a letter apologizing for the typo that appeared on the law firm's brochures
관계대명사의 생략은 목적격관계대명사나 주격관 계대명사 + be 형용사류 문장에서되는걸로 알고있는데 그럼 apologizing 앞에 that is 이나 which is가 생략되었고 apologize가 자동사라서 현재분사형으로 나온건가요?