What is gerundival attraction and how do I translate it?

"Gerundival attraction" is a fancy term for the process by which a gerund turns into a gerundive. A reminder: a gerund is one of those words that has -nd- in it (e.g., faciendum, "doing"); a gerundive is a word that also has that -nd- in it, but functions as an adjective that expresses obligation (e.g., faciendus-a-um, "x needing to be done"). We see gerunds in e.g.: ars scribendi, "the art of writing" (genitive of the gerund scribendum). If we want to say something like "the art of writing a letter" (or "the art of letter-writing"), you may think we just need to add the accusative of "letter", epistulam: ars epistulam scribendi . This is logically correct, but grammatically wrong: Latin asks us to shift the noun object (epistulam) into the CASE of the gerund (genitive): epistulam > epistulae. It then asks us to make the gerund (scribendi) agree with the GENDER (and NUMBER) of the noun (in this case fem. sing.),* while retaining its original case (genitive): scribendi > scribendae: at this point, the gerund is functioning and has become a gerundive, describing the noun it is used with. In short, we write: ars epistulae scribendae. Literally, this means "the art of a letter that needs to be written" (recall the sense of obligation that the gerundive usually has), but in English we just say "the art of writing a letter". So, a tip for you: when translating, don't worry about having to capture the sense of the gerundive, and instead just treat the gerundive as a gerund. *Another ex., where you can see the change from sing. to pl.: imperator nuntium misit ad civos monendos ("the general send a messenger to warn the citizens"). Before gerundival attraction, this would've been imperator misit ad civos monendum [=gerund], but because the gerund has a direct object (civos), Latin insists on a change: civos, the object, must go into the CASE of the gerund, i.e. the accusative (conveniently it already is in that case, so stays the same); monendum, the gerund, must go into the GENDER + NUMBER of the object, i.e. masc. pl. [2nd declension], taking us to monendos.