Why do you only forget certain things during amnesia but remember common knowledge and how to speak?

I know it's not always the case but why is it like that when it does happen?Why do you only forget certain things during amnesia but remember common knowledge and how to speak?
that's short term memory compared to long term memory. you'll most likely remember your name, but you won't remember how you fractured your skull or what day of the week it is. it's the things that are hard-wired into your character, personality, and the things you picked up when you were young that you usually remember.Why do you only forget certain things during amnesia but remember common knowledge and how to speak?
i think that is called selective amnesia, that person actually don't want to remember certain things because that memory may be attached to some unpleasant incident that is why the subconscious mind wants to erase it.