I’ll never forget the days when I worked together with you.
= I’ll never forget the days (when) I worked together with you.
= I’ll never forget (the days) when I worked together with you.
→ 선행사(the days)를 생략할 경우, when이하는 동사(forget)의 목적어인 명사절이 된다.
= I’ll never forget the days that I worked together with you.
→ when을 관계부사 that으로 대신할 수 있다.