Semalam, saya menonton film dokumenter di televisi.

Not exactly.

• malam = night
• semalam = last night

In this sentence, semalam gives the time of the action, so it means last night. The prefix-like element se- changes the meaning here.

Be careful: in some contexts, semalam can also refer to yesterday evening/night, depending on the situation, but last night is the usual translation.

Indonesian often puts time expressions near the beginning of a sentence, especially when setting the scene.

So:

• Semalam, saya menonton film dokumenter di televisi.

feels very natural.

But Indonesian word order is fairly flexible. You could also say:

• Saya menonton film dokumenter di televisi semalam.

That also makes sense. Putting semalam first gives it a little more emphasis, similar to English Last night, I watched...

The comma is used because Semalam is a fronted time expression. It helps separate the time phrase from the main clause.

So this is natural in writing:

• Semalam, saya menonton film dokumenter di televisi.

Without the comma, it may still be understandable, especially in informal writing, but the comma makes it cleaner and easier to read.

Indonesian verbs do not change form for tense the way English verbs do.

So menonton can mean:

• watch
• am watching
• watched
• will watch

depending on context.

In this sentence, the word semalam tells you the action happened in the past, so menonton is understood as watched.

This is very common in Indonesian: time words do the job that verb tense does in English.

Tonton is the root word.
Menonton is the active verb form built from that root.

A simple way to think about it:

• tonton = the base idea of watch
• menonton = to watch / watching / watched as a normal active verb

The prefix meN- is very common in Indonesian and often forms active verbs.

So in a basic sentence with a subject doing an action, menonton is the form you expect:

• Saya menonton film. = I watched a movie.

Using just tonton here would sound incomplete or unnatural in standard Indonesian.

In Indonesian, the main noun usually comes first, and words that describe it come after.

So:

• film dokumenter = documentary film
• literally, something like film documentary

This noun + modifier pattern is very common in Indonesian:

• rumah besar = big house
• buku baru = new book
• film dokumenter = documentary film

English often puts modifiers before the noun, but Indonesian usually puts them after.

Indonesian does not normally use articles like English a, an, or the.

So:

• film dokumenter can mean a documentary film or the documentary film

The exact meaning depends on context.

If you really want to make something explicitly one, you can add a word like:

• sebuah film dokumenter = a documentary film / one documentary film

But in ordinary sentences, leaving it out is completely normal.

Here di is a preposition meaning in / at / on, depending on context.

So:

• di televisi = on television / on TV

In this sentence, it tells you where the film was watched, or more naturally in English, the medium:

• I watched a documentary on television.

This di is written separately because it is a preposition.

Compare:

• di televisi = on television → di is separate
• ditonton = a verb form with the prefix di- → di- is attached

That spelling difference is important in Indonesian.

Yes, absolutely.

Both are natural:

• di televisi = a bit more full/formal
• di TV = very common in everyday speech and writing

So these both work:

• Semalam, saya menonton film dokumenter di televisi.
• Semalam, saya menonton film dokumenter di TV.

Yes, you could, but the tone changes.

• saya = neutral, polite, standard
• aku = more informal, personal, casual

So:

• Semalam, saya menonton film dokumenter di televisi.
  sounds neutral and standard

• Semalam, aku menonton film dokumenter di televisi.
  sounds more casual and conversational

For learners, saya is often the safest choice until you are comfortable with levels of formality.

It is completely natural.

The sentence has a very normal Indonesian structure:

• Semalam = time
• saya = subject
• menonton = verb
• film dokumenter = object
• di televisi = place/medium

So it sounds like ordinary, correct Indonesian.

A native speaker might also vary it slightly, for example:

• Semalam saya nonton film dokumenter di TV.

That version is more casual because nonton is the everyday shortened form of menonton. But your original sentence is perfectly good standard Indonesian.