What is the role of who in this sentence?

2. Subject in attributive clause

3. Not to be omitted, not to be omitted in attributive clause

4. When who is the subject, attributive clause starts with who

When who is the object and there is no preposition in advance, then who can be used or whom

When who is the object. At this time, you can only use whom, because with is a preposition, which is an object-object phrase and can only be used in the accusative form.

5、Once upon a time there was a poor boy ，and he lived in Denmark.