Learn Spanish (Latin America): ¿Por qué usaban velas cuando leían en la noche? - Why did you use candles when you read at night?

Because this sentence is talking about a repeated, habitual action in the past. When you use cuando (when) to talk about things you used to do regularly, both the main action (usaban velas) and the time marker (cuando leían) need to be in the imperfect tense.

The translation says "did you use", but couldn't usaban and leían also mean "they"?

Yes! Without a pronoun like ustedes (you all) or ellos (they), it could mean either one. The English translation assumes "you all" based on an invisible context, but "Why did they use candles when they read at night?" is also a completely perfect translation.

Is it normal to say en la noche? I thought "at night" was por la noche.

Both are correct, but en la noche is very common and natural in Latin America for saying "at night." You will also see de la noche when giving a specific time (like "eight at night" - las ocho de la noche), but for a general time of day like this, en la noche is perfect.

Why does leían have an accent mark on the "i"?