Is the "det" necessary when using an adjective with the definite form of a noun? Dyret = the animal, Det lille dyret = the little animal? If so, is det the only word that can be used (or I guess den for masculine/feminine words)? Thanks!
Yes, that's generally how we write it. The placement shift works like in English "The animal" becomes "The little animal" rather than "Little the animal", it's just that we keep the definite suffix on the noun as well.
You'll see a few exceptions dropping the suffix in proper nouns (Det kongelige slott), and occasionally find the same in poetry (to keep with the metre), as well as in old (Danish inspired) prose.
The dropped suffix is also used very sparingly in modern prose, but you run the risk of sounding either uneducated or pretentious - or both - so it's not something I'd advise learners to try their hand at. We will not allow it in this course.
You're correct in assuming that it's "den" for m/f nouns. :)
Thanks for all the help you give in these comment sections, Linn! Your answers are always so helpful and you seem to have that rare gift of being a native-speaker that actually understands wherein the problems can lie for learners.