This explains it: http://french.about.com/od/grammar/a/pronouns_stressed.htm
In the first two examples I gave they're stressed pronouns because they come after a preposition (avec)
Duolingo accepts (most) British English versions of answers. Such as 'have you got' and 'trousers' and spellings like 'colour' and 'recognise'. I'm from SE England and we would never use the word 'dinner' to describe lunch. But then again, Duolingo refuses to accept 'luncheon' too. It's just something we'll all have to put up with. I do wish, however, they would accept 'whilst' and 'amongst' because use of 'while' and 'among' is somewhat jarring.
If you mean they were not included in the drop-down hints, then it will never work that simply. "Je dois" (conjugated from devoir) can mean "I must, I need to, I have to or even I owe".