When I first encountered this, I just memorized the genitive form of the word as if it were a completely different word:
animus, -î: spirit, soul, mind
animî, -ôrum: high spirits, pride, courage
Why is the genitive form presented with a different meaning from the nominative (this happens several times throughout the book…)?
it is not the genitive, rather, a plural form. Some words allow different meanigs in the plural form.
for instance:
castrum, i = fort
castra orum = millitary basecamp
So basically, whenever I see the word like:
animi, animorum, animis, or animos, it means pride/courage?
not always. as i said, thay allow other meaning as if it were a new word, but it does not mean that thaey have not the normal plural.
see littera, ae for examples.
the context shall make it clear.
So basically, whenever I see a word presented in two forms, like the example I first posted, I can just memorize all the meanings for the one word (I don’t have to make separate flash cards for the plural and singular)?
memorize them as they are given. if the “special” meanigs are to you presented first, memorize so, later bother with the “normal”. vice versa
You ought to treat them as separate words, because its just what they are. though it is true that these words “plural only” have their meanings related to the word of origin. it is no big deal, really.
Thanks for the help; I understand now. I have one more question: I read somewhere that it’s best to learn foreign language vocabulary from context (by looking it up in the dictionary) after having learned one hundred words. How am I to do this if I’m studying from Wheelock’s?
Thanks again.
sorry but i cannot answer that, i did not used wheelock, (i repplied because it was a grammar question 
) someone should come here in your help.
Wheelock’s has supplemental reading book.
Also, there aren’t merely 100 words, but 1400 words that must be learned. You can read them here.
Once you got those, you can read through most texts with only consulting the dictionary every now and then.
So basically, I just make flash cards for each of the words, learn all of them, get through Wheelock’s, and I’m in the intermediate level?
Long way to go.. but I’ll get there!
Similarly, we have a double meaning given to a plural of that same word in English:
spirit- soul, ghost
spirits- alcohol