Why is there no O/A before carta? Thank you for THE letter? They drilled this into us for all these lessons then don't explain why it's not there.
(And I looked again after reading the discussion ... the hint for pela says only "for" and "by" no "for the", which makes it even harder to learn.)
there is an O/A: think of pela as "por a"....and pelo as "por o" .... but it's smoother/easier to say pela, pelo so they abbreviated it into one word...It's sorta like how when they sing Happy birthday, they say "Parabens pra voce"--a shortened para to pra... sometimes a slang equivalent is somewhat built into the language.
No, obrigado/a always depends on who is saying it. I am a woman, so I would say obrigada to a man, and also obrigada to a woman, or to a group of men, or a group of women or a mixed group. It is ALWAYS obrigada because I am a woman. A man would always say obrigado, to a woman, obrigado to a man, or anyone else. DuoLingo is wrong here because it uses a female voice for both obrigado, which should be a male voice saying it) and obrigada, which only females would say.
It should have been accepted - please report it as being correct (using the "Report a Problem" function that shows up after you've completed an exercise and Duo gives you either a red or green screen) next time you run into it.
Thanks in advance, and good luck with your progress :)