Learn Spanish (Latin America): Él iba a trabajar en la ciudad, pero decidió hacerse ermitaño. - He was going to work in the city, but he decided to become a hermit.

We use hacerse for changes that are deliberate, voluntary, or require active effort. Becoming a hermit (ermitaño) is a conscious lifestyle choice that he decided to make. Volverse is generally used for involuntary, natural, or psychological changes (like becoming grumpy or going crazy).

Why isn't there an un before ermitaño? In English it is "a hermit".

When using hacerse to talk about adopting a profession, religion, or specific lifestyle, you usually drop the article (un or una). You say hacerse ermitaño just like you would say hacerse médico (to become a doctor).

Why is iba in the imperfect tense but decidió is in the preterite?

iba a trabajar is in the imperfect because it describes a past plan or ongoing intention (he was going to work) setting the background scene. decidió is in the preterite because his decision was a specific, completed event that interrupted or changed that plan.