"We could not do anything for her."
Translation:Wir konnten nichts für sie tun.
I believe this is because prepositional phrases generally go after any verbs, nouns, or pronouns.
Sorry this isn't a very good explanation. I've never actually seen a rule for this. This is just an observation of mine. Maybe a native could confirm? For now, you'll at least have a bit of an idea as to why this is.
One of my German teachers said that, "könnten", the Konjunktiv II Form, is not used for the 1st persons, meaning; it doesn't make sense to say "What could I do for you?" rather "What can I do for you?". I think the explanation would be, because you should know what you are capable of doing, even for politeness you wouldn't ask it that way.