Learn Hardcore Indonesian: Saya memutar video pendek di komputer.

Memutar is a verb that literally comes from the base verb putar, which means to turn / to rotate something.
With the prefix me-, memutar often means:

to turn something (physically), or
to play media (a video, a song, a film) by “turning”/running it.

So in this sentence, memutar means to play (a short video) on a device.
The me- prefix makes it an active verb: Saya memutar… = I play / I am playing…

Why is the prefix me- written as mem- in memutar, not just meputar?

Indonesian adjusts the me- prefix depending on the first letter of the root word:

Before p, me- becomes mem-, and the p is usually dropped.

Base form: putar
With me-: me- + putar → memutar (not meputar and not *mep*utar).

So memutar is just the regular, phonetic form of me- + putar.

What is the difference between memutar video and menonton video?

Both can appear in similar contexts, but they focus on different actions:

memutar video = to play/run a video (focus on operating the device or starting the video).
menonton video = to watch a video (focus on watching with your eyes).

In your sentence, Saya memutar video pendek di komputer, the focus is on the act of playing the video on the computer.
If you wanted to emphasize that you are watching it, you’d say: Saya menonton video pendek di komputer.

Why is it video pendek, not pendek video?

In Indonesian, adjectives normally come after the noun:

video pendek = short video
rumah besar = big house
buku baru = new book

So video (noun) comes first, then pendek (adjective).
Putting pendek before video (pendek video) would sound wrong or at least very unnatural.

Why is there no word for a in video pendek? Why not sebuah video pendek?

Indonesian does not have a direct equivalent of the English articles a / an / the.
You can add sebuah (a classifier) to emphasize “one (single) thing”:

Saya memutar sebuah video pendek di komputer. = I play a short video on the computer.

But in everyday speech, sebuah is often omitted unless you really need to highlight that it is one item.
So Saya memutar video pendek di komputer is completely natural and usually understood as I (am) playing a short video on the computer.

How do we know if this is past, present, or future? There is no tense marker.

Indonesian verbs generally do not change form for tense.
Saya memutar video pendek di komputer can mean:

I played a short video on the computer. (past)
I am playing a short video on the computer. (present)
I will play a short video on the computer. (future, but less common without extra words)

The time is usually shown by context or by adding time words:

tadi (earlier): Tadi saya memutar video pendek di komputer.
sekarang (now): Sekarang saya memutar video pendek di komputer.
nanti (later): Nanti saya memutar video pendek di komputer.

Could the sentence mean I play short videos on the computer (plural)?

Yes, it can. Indonesian usually does not mark plural on the noun.
video can mean video or videos, depending on context.

Saya memutar video pendek di komputer.
→ could be I play a short video on the computer
→ or I play short videos on the computer.

If you really want to stress plural, you could say:

Saya memutar beberapa video pendek di komputer. = I play several short videos on the computer.
Saya memutar banyak video pendek di komputer. = I play many short videos on the computer.

What does di mean in di komputer? Is it “in”, “on”, or “at”?

Di is a general preposition of location, often translated as in / on / at, depending on context.
In di komputer, the most natural English translation is on the computer:

di rumah = at home
di meja = on the table
di komputer = on the computer

So Saya memutar video pendek di komputer is best read as I play a short video on the computer.

Can I say pada komputer instead of di komputer?

In modern everyday Indonesian, di is the normal preposition for physical location: di rumah, di sekolah, di komputer.
Pada is used more:

in formal writing,
with abstract objects (e.g. pada kesempatan ini = on this occasion),
with pronouns in formal style (e.g. pada saya, pada mereka).

Pada komputer is grammatically possible but sounds unnatural for ordinary speech; di komputer is what people actually say.

Is di here a preposition or a prefix like in passive verbs (e.g. dibaca)?

Here, di in di komputer is a preposition of place, not a prefix.
You can tell because:

As a preposition, di is separate from the noun: di komputer, di meja.
As a passive prefix, di- attaches directly to a verb with no space: dibaca, ditulis, dikerjakan.

So in your sentence:

memutar = active verb with me- prefix
di komputer = prepositional phrase (di
- noun)

Can I omit Saya and just say Memutar video pendek di komputer?

Spoken Indonesian often drops subject pronouns if the subject is clear from context, but:

A bare Memutar video pendek di komputer. sounds incomplete or like a sentence fragment, unless it is written as a note or instruction (e.g. in a manual: Memutar video pendek di komputer. = “Play a short video on the computer.”).

For normal, full sentences, you would usually keep Saya:

Saya memutar video pendek di komputer.

In very casual conversation, people more often change the pronoun (e.g. to aku or gue) than drop it entirely.

Are there more casual or natural alternatives to Saya memutar video pendek di komputer?

Yes. Depending on the context and how colloquial you want to be:

Saya menonton video pendek di komputer. = I watch a short video on the computer.
Saya lagi nonton video pendek di komputer. (colloquial) = I am watching a short video on the computer (right now).
Aku nonton video pendek di komputer. (more casual pronoun aku).
Gue nonton video pendek di komputer. (Jakarta slang, very casual).

Memutar sounds a bit more like talking about the action of playing a file (e.g. in a technical or descriptive context), while nonton focuses on watching.

Can I change the word order and say Saya di komputer memutar video pendek?

You can, and it is grammatically possible, but it sounds a bit marked or unusual in everyday speech.
The most natural word order is:

Saya memutar video pendek di komputer. (Subject – Verb – Object – Place)

Moving di komputer earlier might be used for emphasis or in special contexts, but as a learner you should stick to:

[Subject] + [Verb] + [Object] + [Place]
→ Saya memutar video pendek di komputer.