Mi hermano se adaptó pronto a las gafas.

Because in this meaning, Spanish normally uses the pronominal verb adaptarse a.

• adaptar algo = to adapt something
  
  • Adaptaron el libro al cine. = They adapted the book for cinema.
• adaptarse a algo = to adapt to something / to get used to something
  
  • Mi hermano se adaptó a las gafas.

So here se is not really himself in a literal reflexive sense. It is part of the verb pattern adaptarse a.

Here, se is part of the verb adaptarse. It shows that the subject is the one undergoing the adjustment.

So:

• Mi hermano se adaptó... = My brother adapted / got used to...
• not literally My brother adapted himself... in natural English

Many Spanish verbs work this way and are best learned as whole units:

• acostumbrarse a = to get used to
• adaptarse a = to adapt to
• enfrentarse a = to face

The accent mark shows the stress and also helps distinguish this form.

adaptó is the third person singular preterite form:

• yo adapté
• tú adaptaste
• él/ella/usted se adaptó

Without the accent, adapto, it would sound different and would correspond to a present-tense form:

• yo adapto = I adapt

So adaptó tells you he/she adapted.

The preterite is used because the sentence presents this as a completed change or achievement: he got used to the glasses.

Mi hermano se adaptó pronto a las gafas suggests:

• there was a period of adjustment
• and he successfully adjusted, fairly quickly

If you used the imperfect, se adaptaba, it would sound more like:

• he was adapting
• he used to adapt
• the process was ongoing or being described in the background

So the preterite fits the idea of a completed adjustment.

Because the verb pattern is adaptarse a algo.

So you say:

• adaptarse a las gafas
• adaptarse al clima
• adaptarse a una nueva ciudad

That a belongs to the verb, just like in English we say adapt to something.

In Spanish, articles are often used where English would use a possessive or no article at all, especially with clothing, body parts, and personal items when the owner is obvious from context.

So a las gafas here naturally means:

• to the glasses
• but in good English we would usually say to wearing glasses or to his glasses, depending on context

Spanish often prefers:

• Se acostumbró a las gafas. rather than
• Se acostumbró a sus gafas.

Using sus gafas is possible, but it can sound more specific or contrastive.

In Spain, gafas is normally a plural noun meaning glasses.

Just like in English, the item has two lenses, so it is treated as plural:

• las gafas
• unas gafas nuevas

In Spain, gafas is the usual everyday word for glasses. In other Spanish-speaking regions, people may also say:

• lentes
• anteojos

Here pronto means something like soon or before long, but in natural English this sentence is often translated as quickly because that sounds better with adapted.

So:

• se adaptó pronto = he adapted soon / he got used to it quickly

The exact English wording depends on style, but the Spanish idea is that the adjustment happened after a short time.

That is a very natural position for adverbs in Spanish.

• Mi hermano se adaptó pronto a las gafas. = very normal
• Mi hermano pronto se adaptó a las gafas. = also possible, but slightly different in rhythm/emphasis

Putting pronto after the verb sounds neutral and natural here.

Yes. Spanish often drops subject pronouns and even noun subjects when the context is clear.

So you could say:

• Se adaptó pronto a las gafas.

That would still be grammatical if everyone already knows you are talking about your brother.

But Mi hermano is included here to make the subject explicit, which is very common.

No, not in standard Spanish for this meaning.

adaptar without se usually needs a direct object:

• Adaptaron el sistema.
• Adaptó la máquina.

But when the meaning is to adapt to or to get used to, you need adaptarse a:

• Mi hermano se adaptó a las gafas.

So adaptó a las gafas is not correct here.

Yes, but they are close.

• adaptarse a = to adapt to, adjust to
• acostumbrarse a = to get used to

In many contexts, both work:

• Mi hermano se adaptó a las gafas.
• Mi hermano se acostumbró a las gafas.

A small nuance:

• adaptarse can sound a bit more like adjusting successfully to a new situation
• acostumbrarse focuses more on becoming used to something through habit or time

In this sentence, either would be natural.

Yes, that is understandable and natural. It means becoming accustomed to wearing glasses.

That said, depending on the context, speakers might also choose:

• acostumbrarse a las gafas
• acostumbrarse a llevar gafas

The version with llevar gafas makes it extra clear that the person is getting used to wearing glasses:

• Mi hermano se acostumbró pronto a llevar gafas.

But the original sentence is perfectly fine.