Dear Latin Teacher,
Please explain why in the first sentence "quae" is used (but not "quas") and in the second sentence "quas".
1) Accepi omnes epistulas quae a te missae erant.
2) Hannibal copias suas trans Alpes quas nemo unquam cum copiis ante eum superaverat in Italiam traduxit.
In the first sentence, quae is the subject of missae erant, so it must be nominative.
In the second sentence, quas is the direct object of superaverat, so it must be accusative.
Translations: 1. I have received all the letters which had been sent by you. 2. Hannibal led all his troops into Italy across the Alps, which no one ever before him had overcome with troops.
Relative pronouns get gender and number from the antecedent, but they get case from how they are used in their own clause.
Hope this helps and thanks for asking a Latin teacher.