I have just been practicing with the flashcards and they say that 'sentiste' means 'heard'. Doesn't it mean '(you) felt', or can it mean both things? Likewise 'oscuridad' got translated to 'dark', was I wrong in assuming that it meant 'darkness', or could it be both (or not)?
Likewise, I am experiencing some inconsistencies with the flashcard translations of conjugations. Sometimes they are specific about the form and other times they are not.
Are these 'errors' (perceived or not) intended to provide more understanding, or are they just that?
It depends on the context. "Sentiste" means "you felt" but in a sentence like "sentiste un ruido" you can translate it as "you heard a noise" (can you say in English "you felt a noise"?). The key is that "sentir" in Spanish refers to all senses (you can "sentir" the music, the touch, the cold, the taste, etc).
As for "oscuridad", it is a substantive, not an adjective. So you can say "la oscuridad de la noche" (the dark/darkness of the night) but not "la oscuridad noche" (the dark night). Here you should say "la oscura noche".