Is there a rule governing when to use någon/något in place of en/ett. Grammar books give very little on this
In this sentence from book I'm reading:
...tror jag inte att hon blir någon lämplig hustru åt mig.
I'd have probably written .. blir en lämplig...
So how do we know when to use någon/något in place of en/ett? I'm aware it can mean any, someone, etc, but here it seems just to replace the en of hustru.