Learn Hardcore Indonesian: Teman saya dan pasangannya saling mendukung ketika salah satu merasa stres. - My friend and her/his partner support each other when one of them feels stressed.

teman saya – neutral, slightly formal, very clear.
temanku – more informal, feels a bit more personal / spoken.
teman aku – informal, used in casual speech; aku is the casual I/me.

All of them can mean my friend, but teman saya is the safest, neutral form you can use almost anywhere.

Who does pasangannya refer to here, and what does the ending -nya mean?

Pasangannya here means their partner (the partner of my friend).

Breakdown:

pasangan = partner (can be romantic partner, spouse, or just “pair-mate” depending on context)
-nya = his / her / their / the (a general third-person marker)

So teman saya dan pasangannya = my friend and their partner.

In context, -nya clearly refers back to teman saya. Indonesian often uses -nya instead of repeating a full possessive phrase like pasangan teman saya.

Could pasangannya also mean the partner (without saying whose partner)?

Yes, -nya can sometimes mean the instead of his/her/their.

pasangannya could mean the partner if the context is something like Di tim ini, dialah pasangannya (In this team, he is the partner).

But in your sentence, we already mention teman saya, and then pasangannya comes right after it, joined with dan. The most natural reading is:

teman saya dan pasangannya = my friend and their partner

If it were meant to be my friend and the partner (some other, unrelated partner), Indonesian would usually clarify that in some way.

Why is there no word for their in teman saya dan pasangannya?

Indonesian usually uses -nya instead of a separate word like their or his/her.

So instead of:

teman saya dan pasangannya = my friend and *their partner*

we don’t say something like pasangan mereka here, because -nya attached to pasangan already works as a possessive.

If you really wanted to say my friend and his/her partner explicitly, you could say:

teman saya dan pasangannya (most natural)
teman saya dan pasangan dia (clear, but sounds slightly less smooth)
teman saya dan pasangan‑nya (same as the original, just written with a hyphen for teaching)

Learners just need to get used to -nya carrying the possessive meaning.

What does saling mean, and why do we need it with mendukung?

Saling means each other / mutually.

mendukung = to support
saling mendukung = to support each other / to mutually support

So:

Teman saya dan pasangannya mendukung = My friend and their partner support (someone or something, not clear who).
Teman saya dan pasangannya saling mendukung = My friend and their partner support each other.

Saling is used before the verb to show a reciprocal action between at least two people.

Is there another way to say saling mendukung, like mendukung satu sama lain?

Yes, these are very close:

saling mendukung
mendukung satu sama lain

Both mean support each other.

Nuance:

saling mendukung – shorter, very natural, common in both spoken and written Indonesian.
mendukung satu sama lain – sounds a bit more explicit and slightly more formal or literary; still very natural.

In everyday speech, saling + verb is the go‑to pattern:

saling membantu – help each other
saling menghormati – respect each other
saling mendengarkan – listen to each other

What does ketika mean here, and how is it different from waktu or saat?

Ketika means when (in the sense of at the time that).

In this sentence:

ketika salah satu merasa stres = when one of them feels stressed

You could also say:

waktu salah satu merasa stres
saat salah satu merasa stres

All three (ketika, waktu, saat) often translate as when:

ketika – slightly more formal/neutral, common in writing and careful speech.
waktu – literally time, but used a lot as when, especially in spoken Indonesian.
saat – literally moment, but also used like when, often a bit formal or narrative.

In most everyday contexts, they’re interchangeable here.

Does salah satu mean wrong one? I thought salah means wrong.

Yes, salah usually means wrong, but in the fixed phrase salah satu, it means one (of a group).

salah satu = one of (them/these/those)

Examples:

Salah satu teman saya sakit. – One of my friends is sick.
Pilih salah satu. – Choose one (of them).

So in your sentence:

ketika salah satu merasa stres = when one (of them) feels stressed

You should treat salah satu as a set expression meaning one of.

Why doesn’t the sentence say salah satu dari mereka to mean one of them?

It could say that, and it would still be correct:

Teman saya dan pasangannya saling mendukung ketika salah satu dari mereka merasa stres.

But Indonesian often omits things that are clear from context. Here:

We already know we are talking about two people: teman saya dan pasangannya.
So salah satu is enough; it’s obvious it means one of those two.

Adding dari mereka (“of them”) is more explicit but not necessary. The shorter version is more natural in normal conversation or writing.

Why do we use merasa stres instead of just stres?

Both are possible, with a small nuance difference.

merasa stres = feel stressed
stres (as a predicate) = is stressed / is under stress

So you could say:

ketika salah satu merasa stres – when one of them feels stressed
ketika salah satu stres – when one of them is stressed

In practice, Indonesians use both. Merasa stres emphasizes the subjective feeling a bit more, like English feel stressed. The version without merasa is slightly simpler and very common in speech.

Is stres a common Indonesian word, or is it just borrowed from English?

Stres is a common, fully accepted Indonesian word, borrowed from English stress.

It’s used a lot in everyday language:

Saya lagi stres. – I’m stressed (right now).
Pekerjaan ini bikin saya stres. – This job makes me stressed.

There are also more “native” or descriptive alternatives:

tertekan – under pressure, feeling oppressed
kewalahan – overwhelmed
cemas – anxious
gelisah – restless, uneasy

In your sentence, stres is perfectly natural and very common in modern Indonesian.