Después de leer un capítulo, me gusta resumirlo en dos o tres frases sencillas.

Because after después de, Spanish normally uses an infinitive when the subject stays the same.

So:

• Después de leer un capítulo... = After reading a chapter...
• not después de leo

You use the infinitive here in the same way English uses after reading.

If you want a full clause with a different subject, Spanish usually uses después de que + a conjugated verb:

• Después de que mi hermano lee/leía el capítulo...
  = After my brother reads/read the chapter...

But in your sentence, the person who reads is the same person who likes summarising, so leer is the natural choice.

Después de means after.

In this sentence, it introduces what happens first:

• Después de leer un capítulo = After reading a chapter

It is a very common structure:

• Después de comer = After eating
• Después de estudiar = After studying
• Después de trabajar = After working

So después de + infinitive is a very useful pattern.

Because gustar works differently from English to like.

Spanish structures it more like:

• Something is pleasing to me

So:

• Me gusta resumirlo literally means Summarising it pleases me
• natural English: I like to summarise it

The me means to me.

A few examples:

• Me gusta leer = I like reading
• Te gusta leer = You like reading
• Nos gusta leer = We like reading

So me gusta is the normal way to say I like.

Because the thing that gustar is agreeing with here is the infinitive phrase resumirlo, which counts as a singular idea.

So:

• Me gusta resumirlo = I like summarising it

Compare:

• Me gusta leer = I like reading
• Me gusta nadar = I like swimming

But:

• Me gustan los libros = I like books
• Me gustan estas frases = I like these sentences

Use gusta with:

• an infinitive
• one singular noun
• one whole action/idea

Use gustan with:

• plural nouns

Because object pronouns can attach to an infinitive.

Here:

• resumir = to summarise
• lo = it

So:

• resumirlo = to summarise it

This is very common in Spanish with infinitives, gerunds, and affirmative commands.

Examples:

• quiero verlo = I want to see it
• voy a comprarlo = I’m going to buy it
• estoy haciéndolo = I’m doing it

You could also say:

• me gusta resumirlo

but not usually separate the lo here unless the structure changes.

Lo refers to un capítulo, or more naturally, the chapter/the content of the chapter.

Since capítulo is:

• masculine
• singular

the pronoun is lo.

Compare:

• el libro → lo
• el capítulo → lo
• la historia → la
• los capítulos → los
• las historias → las

So resumirlo means to summarise it, with it referring to the chapter.

Yes. Both are correct, but they sound slightly different.

• Después de leer un capítulo, me gusta resumirlo...
• Después de leer un capítulo, me gusta resumir el capítulo...

Spanish often prefers the pronoun lo here to avoid repeating capítulo. It sounds more natural and less repetitive.

So resumirlo is usually the better choice in this sentence.

Here en means something like in or using.

So:

• resumirlo en dos o tres frases = to summarise it in two or three sentences

It tells you the form or length of the summary.

Similar examples:

• explícalo en español = explain it in Spanish
• dímelo en una palabra = tell me in one word
• resúmelo en pocas líneas = summarise it in a few lines

So en shows the medium, wording, or amount used to express something.

Both can be translated as sentences, but they are not always exactly the same.

In everyday language, frase is very common and natural for learners to use. In this sentence:

• dos o tres frases sencillas

means something like two or three simple sentences.

Technically:

• oración is more grammatical and precise: a sentence/clause with a verb
• frase can be broader and often means a phrase or sentence in general use

In normal conversation, frases sounds perfectly natural here.

Because the adjective has to agree with frases.

Frases is:

• feminine
• plural

So the adjective must also be feminine plural:

• frases sencillas

Agreement patterns:

• un capítulo sencillo = a simple chapter
• una frase sencilla = a simple sentence
• dos capítulos sencillos = two simple chapters
• dos frases sencillas = two simple sentences

That is why sencillas is used.

Because in Spanish, descriptive adjectives often come after the noun.

So:

• frases sencillas = simple sentences

This is the most neutral and common order.

If you put some adjectives before the noun, the meaning can become more stylistic or subjective, but here the normal order is:

• dos o tres frases sencillas

Because Después de leer un capítulo is an introductory phrase, and Spanish often separates that from the main clause with a comma.

So the structure is:

• Después de leer un capítulo,
  me gusta resumirlo...

This comma helps readability. In short introductory phrases, some writers may omit it, but here using it is natural and clear.

Both are possible, but the original order is very natural because it sets the time/context first.

• Después de leer un capítulo, me gusta resumirlo...

This sounds like:

1. first I read a chapter
2. then I like to summarise it

Spanish often places this kind of time phrase at the beginning to frame the sentence.

If you said:

• Me gusta resumirlo en dos o tres frases sencillas después de leer un capítulo

it is still understandable, but the original version sounds smoother and better organised.

Yes. Un capítulo here is general, meaning a chapter in the sense of whenever I read a chapter or after reading a chapter.

It does not point to one specific known chapter.

If it were a specific one, you might say:

• Después de leer el capítulo... = After reading the chapter...

So:

• un capítulo = any chapter / a chapter in general
• el capítulo = a specific chapter

Yes. Tras leer un capítulo is also correct and means the same thing: after reading a chapter.

Difference in feel:

• después de = very common, neutral
• tras = slightly more concise, sometimes a bit more formal or written

So both work, but después de leer is probably the most straightforward choice for everyday Spanish.

It sounds like a habit or a general preference.

• me gusta resumirlo... = I like to summarise it...

This is not talking about one particular moment only. It suggests something the speaker generally likes doing after reading a chapter.

If you wanted a one-time action in a specific situation, Spanish would usually phrase it differently, for example with a past tense:

• Después de leer un capítulo, lo resumí en dos frases.
• After reading a chapter, I summarised it in two sentences.

So the original sentence expresses a routine or usual preference.