Learn Hardcore Indonesian: Sepanjang sore, kami menunggu pelangi setelah hujan reda. - Throughout the afternoon, we wait for a rainbow after the rain lets up.

Sepanjang sore means “throughout the afternoon,” implying the action continued the whole time.
Selama sore is understandable but less idiomatic; you more often see selama dua jam / selama musim hujan. If you do use it with sore, add a determiner like ini/tadi.
Pada sore hari just marks the time (“in the afternoon”) without the “throughout” sense.
Note: sore generally covers late afternoon (roughly 3–6/7 p.m.). A near-synonym is petang (more formal/literary in Indonesian).

Why is it kami, not kita?

Kami excludes the listener (“we, not including you”).
Kita includes the listener (“we, including you”).
Using kita would suggest the person you’re talking to was part of the waiting.

Why menunggu and not tunggu?

Menunggu is the standard active verb form in statements: “to wait (for).”
Tunggu is the base form, very common in imperatives (e.g., Tunggu saya!) and informal speech. A sentence like Kami tunggu pelangi can appear in casual contexts but sounds clipped; Kami menunggu pelangi is neutral and natural.

Does menunggu need a preposition like untuk or akan?

No. Menunggu is a transitive verb; it takes a direct object without a preposition: menunggu pelangi is correct.
Avoid menunggu untuk/akan pelangi; that sounds ungrammatical.

Is pelangi definite or indefinite here? Should I add sebuah?

Bare nouns in Indonesian are often indefinite by default; pelangi here reads as “a (the) rainbow.”
You can say sebuah pelangi to stress “one rainbow,” but it’s usually unnecessary. Most natural is simply pelangi.

Can I move the setelah-clause to the front? Does it change the meaning?

Yes: Setelah hujan reda, kami menunggu pelangi. Same meaning; it just foregrounds the time condition.
You can also say: Sepanjang sore, setelah hujan reda, kami menunggu pelangi. Word order is flexible; choose what you want to emphasize.

Is the comma after Sepanjang sore required?

It’s optional but recommended when an adverbial phrase is fronted: Sepanjang sore, ...
If the sentence gets long (e.g., with a setelah-clause), commas improve readability.

Why reda and not berhenti? What’s the nuance?

Reda/mereda = to abate, let up, subside (it may still drizzle or be lighter).
Berhenti = to stop completely.
So hujan reda can imply the rain eased; hujan berhenti means it stopped.

Can I say setelah hujan mereda or sudah reda?

Yes: setelah hujan mereda is very common. Reda (adjective/verb-like) and mereda (intransitive verb) are both idiomatic.
Sudah is optional: setelah hujan (sudah) reda; with setelah, sudah adds emphasis but isn’t required.

How would I say “as soon as the rain let up”?

Use begitu: Begitu hujan reda, kami menunggu pelangi.
Or ketika (“when”), which is looser in timing: Ketika hujan reda, kami menunggu pelangi.

Is there any ambiguity about what setelah hujan reda modifies?

At the end, it most naturally modifies the time of the waiting: they waited after the rain let up.
If you worry about ambiguity, front it: Setelah hujan reda, kami menunggu pelangi sepanjang sore.

Pronunciation tips for sepanjang, menunggu, reda?

How do we know this happened in the past if there’s no tense marker?

Indonesian doesn’t mark tense on verbs. Time is inferred from context and adverbials.
You can add time words: tadi sore (earlier this afternoon), kemarin sore (yesterday afternoon), or an aspect marker: kami sudah menunggu...

Could I use menanti or menantikan instead of menunggu?

Menanti ≈ “to await,” a bit more literary/poetic: Kami menanti pelangi...
Menantikan is also fine and slightly more formal/intense: Kami menantikan pelangi... You can also say menantikan kemunculan pelangi (“await the appearance of a rainbow”).

Is menunggui a valid alternative?

No. Menunggui means “to watch over/attend” (e.g., menunggui pasien = sit with a patient). It’s not used for “waiting for” something to appear. Use menunggu/menanti/menantikan.