I came across the quotation "Nunc scio quid sit amor" (Virgil, Ecl. VIII.43), and I’d like to know why the subjunctive "sit" is used instead of "est" here. Since it means "Now I know what love is" and not "Now I know what love might be", why wouldn’t "is" be indicative in Latin?
The direct question 'What is love?' has been embedded into another sentence, forming an indirect question. An indirect question 'gives the substance of the question, adapted to the form of the sentence in which it is quoted' (Allen & Greenough, New Latin grammar, §330.2). In Latin, the verb in indirect questions is usually subjunctive, not indicative. However, they are often translated as though the verb were indicative; the use of the subjunctive really just shows subordination.