Learn Hardcore Indonesian: Dia mengganti saluran televisi ketika iklan muncul. - He/She changes the television channel when commercials appear.

Dia is a third‑person singular pronoun for people, meaning he or she.
Indonesian usually does not mark gender in pronouns, so dia is neutral.
You know whether it is he or she only from the context or from what has been mentioned earlier in the conversation.

Why is it mengganti and not just ganti?

The base verb is ganti (change, replace).
The prefix meN- (here realized as meng-) makes it an active transitive verb, mengganti, roughly “to change something / to replace something.”
In standard sentences with a clear subject and object, you normally use mengganti:

Dia mengganti saluran televisi. = He/She changes the TV channel.
Bare ganti is more common in imperatives or informal speech, for example:
Ganti saluran! = Change the channel!

What is the difference between mengganti and mengubah in this context?

Both can be translated as to change, but the nuance is different.

mengganti = to replace one thing with another (switch from A to B).
mengubah = to alter or modify something’s state or form.

For TV channels, you are “replacing” one channel with another, so mengganti saluran televisi is the natural choice.
Mengubah saluran televisi would sound odd, as if you are changing the nature of the channel itself, not which channel you’re watching.

What does saluran televisi literally mean, and can I say it in a shorter way?

Literally, saluran televisi is television channel (saluran = channel, televisi = television).
In everyday speech you will also hear:

saluran TV (very common)
just saluran, if it’s clear you are talking about TV
borrowed English channel, especially in informal contexts.

So these are all acceptable in context:

Dia mengganti saluran televisi.
Dia mengganti saluran TV.
Dia mengganti saluran.
Dia ganti channel. (informal)

Could the sentence also start with Ketika iklan muncul? Is that still correct?

Yes. You can switch the order of the main clause and the time clause:

Dia mengganti saluran televisi ketika iklan muncul.
Ketika iklan muncul, dia mengganti saluran televisi.

Both are grammatically correct and mean the same thing.
Putting Ketika iklan muncul first slightly emphasizes the time/situation.

How is tense expressed here? Does the sentence mean changes, changed, or was changing?

Indonesian verbs normally do not mark tense. Mengganti just means change / replace without a time reference.
The sentence Dia mengganti saluran televisi ketika iklan muncul can be translated, depending on context, as:

He/She changes the TV channel when the commercials come on. (habitual present)
He/She changed the TV channel when the commercials came on. (past event)

You infer the tense from the broader context or from time words like tadi, kemarin, besok, etc., which are not present here.

Why is ketika used here? Could I also use saat or waktu?

Ketika, saat, and waktu can all introduce a time clause and often translate as when.

In this sentence, all of these are possible:

Dia mengganti saluran televisi ketika iklan muncul.
Dia mengganti saluran televisi saat iklan muncul.
Dia mengganti saluran televisi waktu iklan muncul.

Rough differences:

ketika – neutral, slightly more formal or written.
saat – very common in both spoken and written Indonesian.
waktu – more colloquial in this function.

Meaning-wise here, they are effectively the same.

What does muncul mean exactly in ketika iklan muncul?

Muncul means to appear or to show up.
In ketika iklan muncul, it literally means when the advertisements appear.
In English, we would usually say when the commercials come on, so muncul here is the natural Indonesian way to express come on (screen) or show up.

Could I also say ketika ada iklan instead of ketika iklan muncul? Is there a difference?

Yes, you can say:

Dia mengganti saluran televisi ketika ada iklan.

This literally means when there are commercials or whenever there are commercials.
Ketika iklan muncul focuses on the moment the commercials appear.
Ketika ada iklan focuses more on their presence in general.
In practice, for this context (changing channels during ads), both are fine and very natural.

Can I omit televisi and just say Dia mengganti saluran ketika iklan muncul?

Yes. If the context makes it clear you are talking about TV, Dia mengganti saluran ketika iklan muncul is completely natural.
Indonesian often omits information that is obvious from context.
Including televisi just makes it explicit that the saluran is a TV channel.

Is it possible to drop Dia? For example, just Mengganti saluran televisi ketika iklan muncul?

In standard written Indonesian, you normally keep the subject, so Dia is expected.
In very informal speech or notes, you might see or hear sentences without an explicit subject when it is already strongly implied by context, but grammatically a sentence like Mengganti saluran televisi ketika iklan muncul feels incomplete.
So for learner Indonesian, keep Dia (or another clear subject).

Does iklan here specifically mean TV commercials, or can it be any kind of advertisement?

Iklan is a general word for advertisement: print ads, online ads, TV commercials, radio ads, etc.
In the context of a TV, iklan is naturally understood as TV commercials.
If you want to be very explicit, you can say iklan TV, but it’s usually not necessary when you’re already talking about saluran televisi.

Should it be iklan-iklan to show that there are many commercials?

You don’t have to mark plural in Indonesian. Iklan can mean an advertisement or advertisements, depending on context.
Reduplication (iklan-iklan) is possible but often used when you really want to stress many ads as separate units.
In this sentence, Indonesian speakers are happy with the unmarked plural: iklan is understood as ads / commercials.

Is Dia mengganti saluran televisi ketika iklan muncul formal, informal, or neutral?

It is neutral and can be used in both spoken and written Indonesian.
The vocabulary (dia, mengganti, saluran televisi, ketika, iklan, muncul) is standard.
Only small changes (like using waktu instead of ketika, or channel instead of saluran) would make it sound more clearly informal.