Why is it that sometimes Duolingo will translate the definite article as the posessive whilst in this case it marked me as wrong for translating as 'his hypothesis'?
I think that to be his, the original sentence would have to use "a hipótese dele" or "a sua hipótese". Since it only says "a hipótese," the closest translation is then "the hypothesis". I hope it helps! =)
This would still be "The student copies the hypothesis". Unless it was "O aluno copia sua hipótese / a hipótese dele".
Like Danmoller commented, it gets confusing because of sentences like "Ele machucou a perna" = "He hurt his leg". It sounds redundant, in Portuguese, to say "Ele machucou a perna dele", because, if he had hurt somebody else's leg, then it would have been specified; since it wasn't, you're supposed to assume it was his own leg. But "He hurt the leg" is pretty odd in English C:
The Brazilian article will only mean "his" for sure if it's the most obvious meaning, like what happens with body parts.
But "a" in "ele machucou a perna" is not. However, it's translated in its best with "his".
That was his question.
Sometimes Portuguese phrases will not have a possessive pronoun but the listener will be aware that the most probable meaning indicates possession:
"Ele coloca a mão no bolso" = "He puts his hand in his pocket"
There is no possessive pronoun for "the hand" (a mão), nor for "the pocket" (no bolso).
But the translation to english should include the possessive pronoun because it works differently, and if you were to translate it literally as "He puts the hand in the pocket" there will be an ambiguity not present for the portuguese listener.