Learn Spanish (Spain): ¿Qué bebían en el siglo quince? - What did they drink in the fifteenth century?

The imperfect tense describes habits, routines, or ongoing situations in the past. Asking what people used to drink over the course of a whole century describes a general, ongoing habit, rather than a single completed action.

What is the infinitive of bebían and how does its ending work?

The infinitive verb is beber (to drink). For regular -er and -ir verbs in the imperfect, you drop that ending and add the -ía endings. For the "they" form, the ending is -ían, making bebían. Notice that the i keeps its accent mark in all forms!

The translation says "fifteenth century," but quince means "fifteen". Why doesn't it use an ordinal number like "fifteenth"?

In Spanish, ordinal numbers (like first, second, third) are rarely used after the number ten. For centuries starting from the 11th century, Spanish uses regular counting numbers. So "the fifteenth century" is literally phrased as "the century fifteen" (el siglo quince).

Where is the word for "they" in this sentence?

Spanish usually drops words like ellos or ellas (they) because the verb ending already gives away the subject. The -n at the end of bebían clearly indicates that "they" or "you all" are doing the action, so an extra pronoun isn't needed here.