Learn Hardcore Turkish: Şimşek çakınca gök gürültüsü duyuyoruz.

It forms a time clause meaning when/whenever (something happens) or sometimes as soon as. Here, çak-ınca = çak (flash/strike, as in lightning) + -ınca (when) → when [lightning] flashes. The vowel changes by harmony:

After a/ı: -ınca (çakınca)
After e/i: -ince (gelince = when [he] comes)
After o/u: -unca (vurunca = when [he] hits)
After ö/ü: -ünce (görünce = when [he] sees)

Could I say Şimşek çaktığında instead of Şimşek çakınca?

Yes. -dığında/-diğinde (built from -DIK + possessive + locative) also means when. Nuance:

çakınca is a bit lighter and common in speech, often general/whenever.
çaktığında can feel slightly more specific or formal. Both are correct: Şimşek çaktığında/çakınca gök gürültüsü duyuyoruz.

Why is it duyuyoruz (present continuous) and not duyarız (aorist)?

Both are possible, with nuance:

duyarız (aorist) = general or habitual truth: “we (typically) hear.”
duyuyoruz (present continuous) = “we are hearing (now),” but is also often used in Turkish for repeated cause-effect statements in everyday speech. For a textbook generalization, Şimşek çakınca gök gürültüsü duyarız is slightly more “rule-like.” Your original is natural too.

Why isn’t there an accusative ending on gök gürültüsü? When would it be gök gürültüsünü?

Turkish marks a definite direct object with accusative. Here it’s indefinite/general (“thunder” in general), so no accusative:

Indefinite: gök gürültüsü duyuyoruz = we hear thunder.
Definite/specific: gök gürültüsünü duyuyoruz = we hear the thunder (that specific thunder now).

What exactly is gök gürültüsü morphologically?

It’s an indefinite noun compound (belirtisiz isim tamlaması):

gök (sky) + gürültü (noise) + 3rd person possessive -sü → gök gürültüsü = “sky-noise,” i.e., thunder. In a definite “of”-phrase you’d mark the first word in genitive: göğün gürültüsü = the noise of the sky (more literally possessive).

Is it ever written as one word (gökgürültüsü)?

Standard modern spelling is two words: gök gürültüsü. You may see one-word or hyphenated forms in informal writing, but the accepted form is the two-word compound.

How do I pronounce the tricky letters here (like ş, ç, ı, ö, ü)?

ş = “sh” (English “shoe”): Şimşek ≈ “shim-shek”
ç = “ch” (English “cheese”): çakınca ≈ “cha-kɯn-ja” (that ı is the dotless i)
ı (dotless i) = a close back unrounded vowel, like the unstressed ‘a’ in “sofa”: -ınca ≈ “-ɯn-jah”
ö ≈ German/French “ö/œ” (rounded front vowel): gök ≈ “gœk”
ü ≈ German “ü” / French “u”: gürültüsü ≈ “gyü-rül-tü-sü”

Why is the word order “... thunder hear” instead of “hear thunder”? Can I move things around?

Turkish is verb-final by default, so objects usually come before the verb: gök gürültüsü duyuyoruz. The time clause (Şimşek çakınca) naturally comes first. You can reorder for emphasis, but the given order is the most neutral. Another very natural variant: Şimşek çakınca gök gürültüsünü duyarız.

Do I need a comma after Şimşek çakınca?

It’s optional but common to place a comma after an initial adverbial clause: Şimşek çakınca, gök gürültüsü duyuyoruz. Without the comma is also fine in everyday writing.

What’s the difference between şimşek and yıldırım?

şimşek = lightning (the flash/light in the sky). Collocation: şimşek çakmak (lightning flashes).
yıldırım = a lightning bolt/strike (especially one that hits the ground). Collocation: yıldırım düşmek (a lightning bolt strikes/falls). Both are “lightning” in English, but Turkish distinguishes the flash vs. the strike.

Why is there no biz (we) in the sentence?

Turkish is pro-drop: person is marked on the verb. duyuyor-uz already encodes “we.” You can add Biz duyuyoruz for emphasis or contrast (“we, not others”), but it’s not required.

Could I say duyabiliyoruz to mean “we can hear”?

You can, but it changes the meaning:

duyuyoruz = we (actually) hear (it).
duyabiliyoruz = we are able to hear (it), focusing on ability/possibility. For a straightforward cause-effect, duyuyoruz/duyarız is more natural.

How do I say “It is thundering” in Turkish, without mentioning “we hear”?

Use the verb gürlemek (to thunder):

Right now: Gök gürlüyor.
Generic/habitual: Gök gürler. Past: Gök gürledi.

How is duyuyoruz built?

duy- (hear) + -uyor/-iyor (present continuous) + -uz (1st person plural) → duy-uyor-uz = duyuyoruz. Because the stem already ends with y (duy), you don’t add an extra buffer; it just flows into -uyor.