Learn Hardcore Malay: Saya simpan fail audio di komputer riba saya.

simpan – root verb, often used in everyday, neutral sentences.
menyimpan – the meN- form of simpan, often a bit more formal or process-focused, like to be keeping / to store / to put away.

In this sentence:

Saya simpan fail audio di komputer riba saya.
Saya menyimpan fail audio di komputer riba saya.

Both are grammatically correct and mean essentially the same thing.
Saya simpan... sounds a bit more direct and conversational; Saya menyimpan... can sound slightly more formal or descriptive, especially in writing.

How do I know if this is past, present, or future? There is no tense marking on simpan.

Malay verbs do not change form for tense. Simpan stays the same in:

I saved the audio file
I save the audio file
I will save the audio file

To show time, Malay uses time words or aspect markers:

Tadi saya simpan fail audio di komputer riba saya.
= Earlier / just now I saved the audio file on my laptop.
Sekarang saya simpan fail audio di komputer riba saya.
= Now I am saving / I save the audio file on my laptop.
Nanti saya akan simpan fail audio di komputer riba saya.
= Later I will save the audio file on my laptop.

So simpan itself is tenseless; context and time expressions tell you when.

Is saya both I and my here? Why is it repeated?

Yes:

First saya (at the start) = I (subject)
Second saya (after komputer riba) = my (possessive)

Malay does not have separate I / my forms the way English does. The same pronoun saya can function as:

Saya penat. – I am tired.
Buku saya. – my book.

In komputer riba saya, the pattern is:

noun + pronoun
komputer riba (laptop) + saya (I) = my laptop

The repetition is normal and not redundant in Malay.

Could I just say Saya simpan fail audio di komputer riba without saya at the end?

You can, but the meaning changes slightly:

Saya simpan fail audio di komputer riba saya.
= I save the audio file on my laptop (clearly your own).
Saya simpan fail audio di komputer riba.
= I save the audio file on a / the laptop (not explicitly yours; it could be any laptop, depending on context).

In spoken Malay, people might still understand it is your laptop from context, but grammatically, removing saya removes the clear possessive meaning my.

What does komputer riba literally mean, and is it the normal word for laptop?

Literally:

komputer – computer
riba – lap (as in on your lap, the part of your body)

So komputer riba = lap computer, just like English laptop.

Yes, komputer riba is the standard term for laptop in Malay, especially in Malaysia. In casual speech, some people also just say laptop, but komputer riba is perfectly normal and widely used.

Why is it fail audio and not audio fail? How does noun order work here?

In Malay, the typical pattern is:

main noun + describing noun / classifier

So:

fail audio = file (of) audio → audio file
fail muzik = music file
fail video = video file

Putting audio after fail is the normal Malay order: type of X often comes after the main noun. It is similar to file audio in some Romance languages (French, Spanish) rather than English audio file.

Does fail here mean the same as the English fail (to not succeed)?

No. In Malay:

fail (noun) – file (computer file, folder of documents)

It is a borrowing from English file, but the spelling changed to fail.

The English verb to fail (not succeed) is usually:

gagal – to fail (e.g. fail an exam)
- Saya gagal ujian. – I failed the exam.

So:

fail (Malay) = file
gagal (Malay) = to fail / failure (in the sense of not succeeding)

Why is there no word for the or a in this sentence?

Malay has no separate words for a / an / the. The noun fail audio can be:

an audio file
the audio file
audio files (in some contexts)

The exact meaning comes from context:

If you are talking about a specific one both people know, it feels like the audio file.
If you are introducing it for the first time, it may feel like an audio file.

If needed, you can make it more explicit using other words:

Satu fail audio – one audio file / an audio file
Fail audio itu – that / the audio file (with itu = that).

Why is the preposition di used? How is it different from pada or dalam?

Di is the basic preposition for physical location: at / in / on depending on context.

In this sentence:

di komputer riba saya – on my laptop (location where the file is stored)

Comparisons:

di – at, on, in (location in general)
- di rumah – at home
- di meja – on the table
dalam – in, inside
- dalam beg – in the bag
  Sometimes used for digital containers too: dalam folder ini – in this folder.
pada – at / on / to, often more abstract (time, date, something non-physical):
- pada hari Isnin – on Monday
- pada pendapat saya – in my opinion

For a file stored on a laptop, di komputer riba saya is the natural choice.

Can I change the word order, like Saya simpan di komputer riba saya fail audio?

That word order is not natural. The usual neutral order is:

Subject
Verb
Object
Location / time

So:

Saya (subject)
simpan (verb)
fail audio (object)
di komputer riba saya (location)

You can move the location phrase di komputer riba saya to the front for emphasis:

Di komputer riba saya, saya simpan fail audio.
= On my laptop, I save the audio file.

But generally, you do not insert the location between the verb and its object as in English I put on the table the book. In Malay, keep fail audio right after simpan.

Is this sentence formal or informal? Can I use it in both speech and writing?

The sentence:

Saya simpan fail audio di komputer riba saya.

is neutral and polite. It is:

Appropriate in normal conversation
Fine in most kinds of writing (email, instructions, simple reports, etc.)

To make it more formal (e.g. technical documentation), you might choose menyimpan:

Saya menyimpan fail audio di komputer riba saya.

To make it more casual (among friends), speakers might change the pronoun:

Aku simpan fail audio dalam laptop aku. (colloquial, uses aku and laptop)

But as given, your sentence is standard, polite Malay suitable almost anywhere.