Right. I have found that, normally, when Duolingo teaches words that can be used in different ways, it will teach both ways in the same skill, even though one way may not belong in that skill.
Surely present tense translation should be accepted here, as we have no way of knowing what the writer intends?
Mhm. In English, to owe (present), owed (simple past), ought to (modal verb and archaic past), should, could, and would are all very nebulous words that are related and interconnect to each other in a weird way that my limited linguistic knowledge is unable to properly explain.
Gratitude=gratidão. Uma pessoa não pode "dever gratidão". Ela pode "sentir gratidão". "Nós sentimos muita gratidão pelo que ele fez". "Dever de gratidão" does exist, although it isn't really used. In case anyone's wondering. Interesting question, Mr David. Asking > assuming. After all, there's that saying about assuming... 8D Especially with such different languages. Por isso, pergunte sempre! :-)