Under this chapter / lesson is given a good description about infixes and endings of verbs in passive forms. Is well to read it.
Agree. This a very useful lesson, clarifies a lot, especially if you try to read new site texts.
I still don't understand why Duo won't put the tips on the mobile version??? I can't think of any reason why it wouldn't be possible to give it to mobile users.
But thank you for pointing me there.