Learn Hardcore Indonesian: Dia menutup buku setelah membaca.

Dia is a third-person singular pronoun that can mean “he” or “she”.
Indonesian pronouns are generally not marked for gender, so dia covers both.
You choose “he” or “she” in English based on context, not from the Indonesian word itself.

How do we know whether to translate dia as “he” or “she” in English?

Indonesian itself does not tell you the gender.
You infer it from context: earlier sentences, the situation, or who is being talked about.
If there is truly no context, you just pick one in English (often “he” or “they”), but native Indonesian speakers themselves are fine with the ambiguity.

Why is there no word for “the” or “a” before buku? How do we know if it is “the book” or “a book”?

Indonesian has no articles like “a/an” or “the”.
The noun buku by itself can be translated as “book / a book / the book”, depending on context.
If you really want to specify “that book”, you can say buku itu; if the context makes it clear, buku alone is enough.

What is the basic form of menutup and membaca, and what does the meN- prefix do?

The dictionary (base) forms are tutup (“close, shut”) and baca (“read”).
Adding the meN- prefix turns them into active verbs:

menutup ≈ “to close (something)”
membaca ≈ “to read (something)”
In many cases, meN- + verb is the normal way to express an active action, especially with a clear object (like buku).

Why is it menutup buku and not just tutup buku?

Menutup buku is the standard, neutral way to say “close the book” in a sentence like this.
The meN- prefix (menutup) emphasizes that it is an active action done by the subject.
You can hear tutup buku in speech (especially commands: Tutup buku! “Close the book!”), but in a full sentence with a subject, Dia menutup buku is more natural and grammatically complete.

In English we would often say “after reading it.” Where is “it” in setelah membaca?

Indonesian doesn’t need to repeat the object if it is already obvious.
Here, because the previous object is buku, setelah membaca is understood as “after reading (the book)”.
If you want to make “it” explicit, you can say:

setelah membacanya (“after reading it”)
setelah membaca buku itu (“after reading that book”).

Where is the past tense in this sentence? How do we know it means “closed” and not “closes”?

Indonesian verbs do not change form for tense (past, present, future).
Menutup can mean “closes”, “closed”, or “is closing”, depending on context.
Dia menutup buku setelah membaca is usually understood as a completed action in the past in a narrative, but you know it’s “closed” only from context or from added time words like tadi (earlier), kemarin (yesterday), etc.

Can we move setelah membaca to the front, like in English: “After reading, he closed the book”?

Yes. You can say:
Setelah membaca, dia menutup buku.
This is natural and common.
The meaning is the same; putting setelah membaca at the beginning just emphasizes the time relationship (“after reading”) a bit more.

Could we say setelah dia membaca instead of setelah membaca? Do we need to repeat dia?

You can say:
Dia menutup buku setelah dia membaca.
or
Setelah dia membaca, dia menutup buku.
Repeating dia is grammatically correct but often sounds a bit heavier and is usually avoided unless you need to avoid ambiguity (for example, when two different people might be involved).
Omitting dia in the second clause (setelah membaca) is very natural when the subject is obviously the same.

What is the difference between setelah and sesudah?

Setelah and sesudah both mean “after” and are largely interchangeable in everyday use.
Setelah is slightly more common in written or formal style; sesudah is also common and fully correct.
You could also say: Dia menutup buku sesudah membaca. with the same meaning.

Can we drop dia and just say Menutup buku setelah membaca?

You can drop dia in very casual contexts if it is totally clear who you are talking about, but the sentence Menutup buku setelah membaca by itself feels incomplete in standard Indonesian.
For clear, neutral sentences—especially in writing or in exercises—you should keep the subject:
Dia menutup buku setelah membaca.

How would we say “He closed the books after reading” (plural) instead of “the book”?

Indonesian usually marks plural with reduplication or with a word like beberapa (“several”), banyak (“many”), or semua (“all”).
You could say:

Dia menutup buku-buku itu setelah membaca. (“He closed the books after reading.”)
Dia menutup semua buku setelah membaca. (“He closed all the books after reading.”)
The plain buku can mean “book” or “books” from context, but if you really want to stress plurality, use buku-buku or a quantity word.