Learn Hardcore Indonesian: Kalau saya terlalu gelisah, saya berhenti sebentar supaya konsentrasi bisa kembali. - When I am too restless, I pause for a moment so that my concentration can return.

Jika saya terlalu gelisah, saya berhenti sebentar... (more formal)
Ketika saya terlalu gelisah, saya berhenti sebentar... (when I’m too anxious, focusing more on time than condition)

Why is saya repeated in both clauses? Can I omit the second saya?

Indonesian often repeats the subject for clarity, especially in writing:

Kalau saya terlalu gelisah, saya berhenti sebentar...

But it’s also natural to drop the second saya:

Kalau saya terlalu gelisah, berhenti sebentar supaya konsentrasi bisa kembali.

Both are correct. Differences:

With repetition (saya ... saya ...): slightly clearer and more careful/explicit.
Without repetition: a bit more compact and casual.

You can even drop the first saya in informal speech if it’s obvious from context:

Kalau terlalu gelisah, saya berhenti sebentar...

What exactly does gelisah mean? How is it different from words like khawatir or cemas?

Gelisah is a general word for being mentally and physically restless or uneasy. It often suggests:

difficulty sitting still
mind not calm
mild anxiety or agitation

Rough comparisons:

gelisah – restless, fidgety, mentally uneasy
cemas – anxious, worried, tense (more about mental worry)
khawatir – worried, concerned (about something specific)
gugup – nervous, jittery (especially before doing something, like speaking in public)

In this sentence:

Kalau saya terlalu gelisah...

it suggests: If I feel too restless/anxious to focus...

What does terlalu add here? How is it different from sangat?

Terlalu means too (excessively):

terlalu gelisah = too anxious / too restless

Sangat means very:

sangat gelisah = very anxious, but not necessarily too much.

So:

Kalau saya terlalu gelisah...
= If I’m too anxious (so that it’s a problem)...

If you said:

Kalau saya sangat gelisah...
= If I’m very anxious... (strong, but not necessarily excessive/problematic)

What does berhenti sebentar literally mean? How is it different from istirahat sebentar?

Literal meanings:

berhenti = to stop
sebentar = for a moment / briefly

So berhenti sebentar = stop for a moment.

Istirahat sebentar uses istirahat = to rest, so it’s more like take a short break.

Nuance:

berhenti sebentar – stop what you’re doing for a short moment (could just pause, not necessarily relax).
istirahat sebentar – rest/relax for a short while, with more sense of taking an actual break.

In this sentence, berhenti sebentar fits well because the idea is simply pausing to recover concentration, not necessarily lying down or relaxing deeply.

What does supaya mean, and how is it different from agar or untuk?

Supaya introduces a purpose or desired result: so that / in order that.

...saya berhenti sebentar supaya konsentrasi bisa kembali.
...I stop for a moment so that my concentration can return.

Comparisons:

supaya – common, neutral: so that / in order that
agar – very similar to supaya, a bit more formal/literary
untuk – usually to / for / in order to before a verb or noun

You could say:

...berhenti sebentar supaya konsentrasi bisa kembali.
...berhenti sebentar agar konsentrasi bisa kembali. (slightly more formal)

If you use untuk, the structure tends to change:

Less natural: untuk konsentrasi bisa kembali
More natural: untuk mengembalikan konsentrasi (to restore concentration)

Why is it konsentrasi bisa kembali instead of saya bisa konsentrasi lagi? Are both correct?

Both are grammatically correct and natural, but they focus slightly differently.

Original:

...supaya konsentrasi bisa kembali.
...so that concentration can return.

Alternative:

...supaya saya bisa konsentrasi lagi.
...so that I can concentrate again.

Nuance:

konsentrasi bisa kembali – treats konsentrasi like a “state” that comes back. Slightly more impersonal/abstract.
saya bisa konsentrasi lagi – focuses on your ability: I can concentrate again.

Both would be easily understood in everyday Indonesian.

Is konsentrasi always a noun, like English concentration, or can it act like a verb in Indonesian?

Konsentrasi is originally a noun (concentration), but Indonesian is flexible: many nouns can function like verbs without changing form.

Examples:

Saya konsentrasi.
= I concentrate / I’m concentrating.
Sulit konsentrasi kalau gelisah.
= It’s hard to concentrate when (you are) anxious.

In your sentence, konsentrasi is clearly used as a noun:

konsentrasi bisa kembali
the concentration can return

But elsewhere you’ll see it used verb-like, especially in spoken language.

There is no past or future tense marking. How do Indonesians know if this sentence refers to past, present, or future?

Indonesian usually does not mark tense with verb changes. Time is understood from:

context
time words (like tadi, kemarin, besok, nanti, sekarang)
the situation of the conversation

Your sentence by itself is a general statement, so it naturally reads as:

When(ever) I’m too anxious, I stop for a moment so that my concentration can come back.
(a general habit, not tied to one specific time)

If you want to be explicit:

Past habit:
Dulu, kalau saya terlalu gelisah, saya berhenti sebentar...
In the past, when I was too anxious, I would stop for a moment...
Future:
Nanti kalau saya terlalu gelisah, saya akan berhenti sebentar...
Later, if I get too anxious, I will stop for a moment...

Is the comma after gelisah required in Indonesian?

In Indonesian, it is standard to use a comma between a dependent clause and the main clause when the dependent clause comes first:

Kalau saya terlalu gelisah, saya berhenti sebentar...

So the comma here is correct and recommended in writing.

If you reverse the order, you usually don’t use a comma:

Saya berhenti sebentar kalau saya terlalu gelisah.

In informal text messages, people often skip commas, but in proper writing the comma in the original sentence is good style.