Why can we use 'dad' and 'mom' standalone, but should use a pronoun or an article with 'father' and 'mother'? For instance when brother and sister are speaking.
- Do you understand mom?
- Do you understand your mother?
'Do you understand mother' sounds like bad English.
The same works for other examples:
- Give dad this book.
- Give your father this book.
'Give father this book' sounds really bad.