I'm not an expert, nor am I a native speaker, but wouldn't it be easier to write the rules for when we DO use "att"? From what I've seen so far you use it with the verbs to be/have and with verbs like tycker om/älskar (there are surely other cases that I'm not aware of), and, as far as I'm concerned, I find it easier to remember when to use it rather than when not to use it.
What do you think?
I think that's mostly Duolingo's template being turtle-heavy. :) They removed some of the template words when the course was first created, such as the penguin - heresy, I know - and added some others.
On plus side I was ecstatic to be able to understand it when turtles were mentioned in a Swedish film I was watching with Mikael Persbrandt in it - I had thought I would never find it useful !
Turtle ecstasy. I await that with great anticipation for the day it happens to me.