Learn Spanish (Spain): Los estudiantes han quedado en preparar todo el temario. - The students have agreed to prepare the whole syllabus.

While quedar on its own usually means to meet up or to arrange to meet, adding the preposition en changes the meaning to 'to agree on' or 'to decide to'. So, han quedado en preparar means they have mutually agreed or committed to prepare it.

Why do we use the infinitive preparar right after en? Could we use a conjugated verb?

After prepositions like en, Spanish always uses the infinitive form of a verb. Because the students are both the ones agreeing and the ones doing the preparing, we just use en + infinitive. If they were agreeing that someone else should prepare it, you would need en que followed by the subjunctive.

Is there a difference between using quedar en and acordar in this sentence?

They are very similar and both mean to agree. However, quedar en implies a mutual, often more informal or conversational arrangement between people. Acordar is slightly more formal and is often used for official agreements.

What exactly is a temario?

A temario is a list of topics (temas) that make up a syllabus, curriculum, or study guide. In Spain, this word is extremely common among students, especially those studying for university exams or civil service exams (oposiciones).