Learn Turkish: Kardeşim bu kılıcı hiç kullanmadı.

This is a great contrast! We use -dı when we know something as a definite fact or witnessed it firsthand. The speaker knows for a fact that their brother never used the sword. If they had merely heard this from someone else, or realized it later by seeing the sword untouched, they would use the reported past: kullanmamış.

Why is the word for sword spelled kılıcı here instead of kılıç?

Two things happen here. First, because the brother used this specific sword, it is a specific direct object and needs the accusative case suffix -ı. Second, adding a vowel suffix to a Turkish word ending in ç usually causes that letter to soften to a c. So, kılıç + ı becomes kılıcı.

Does hiç mean "never"?

By itself, hiç means "ever" or "at all." To mean "never," it must be paired with a negative verb. Notice that the verb kullanmadı has the negative suffix -ma in it (kullan-ma-dı). Using hiç with a negative verb is how you emphasize that an action never happened.

How do we know kardeşim means "my brother" and not "my sister"?

Actually, we don't! The word kardeş simply means "sibling" (usually a younger one) and is completely gender-neutral. The ending -im makes it "my sibling". We translated it as "my brother" here to fit a specific English context, but it could just as easily be "my sister".