"The mouse is afraid of us."
Translation:De muis is bang voor ons.
I might not be correct on this, but I believe it has to do with where you place "ons." In this case, the sentence is structured as subject - auxiliary verb - main verb - preposition - direct object. To say "the mouse is afraid for us," I think you could say it like this (though it's a bit strange sounding): Voor ons is de muis bang voor. (Structured preposition - direct object - auxiliary verb - subject - main verb)
Perhaps a native speaker (or just someone who knows more than I do) could chime in here?
If you want to say "the mouse is afraid for us" using these particular words, it would still be "de muis is bang voor ons", I'm afraid xD...
"Voor ons is de muis bang" (without the second "voor", which makes the sentence ungrammatical) would also still have both meanings available. The change in word order merely switches the emphasis to "voor ons".
The problem here is the ambiguity of the word "voor", which in combination with "bang" can mean "of", but in a more general sense means "for". The intended or most likely meaning in these cases would have to become clear from the context, and if it isn't, then even Dutch native speakers would not be able to tell the two apart with certainty ;). The combination "bang voor" will mostly be interpreted as "afraid of", simply because it is the more common/logical usage. Examples where "afraid for" would be the more likely interpretation because of context/logic are for instance "bang voor je gezondheid" (afraid for your health) or "bang voor je leven" (afraid for your life). Hope this helps!