You may want to go back and reread the explanations on indirect questions. The indirect question uses the interrogative rather than the relative pronoun because the part of the sentence in the subjunctive is actually a question.
In the sentence Nunc scio quid sit amor
, the statement
is Nunc scio
"Now I know" and the question
is Quid est amor?
"What is love?". When you wish to combine them into one sentence, the question portion of the sentence must be put into the subjunctive (i.e. change est
Also, take note that Wheelock specifically says in the second paragraph on page 204:
Wheelock's Latin wrote:The subjunctive in an indirect question is usually translated as though it were an indicative in the same tense (i.e. without any auxiliary [verbs] such as "may" or "might").
Pay attention to how these sentences are translated in the examples. According to this rule, the indirect question Nunc scio quid sit amor
should be translated "Now I know what is
love" or, more properly, "Now I know what love is", without any words such as "may" or "might".
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae