Learn Hardcore Turkish: Film bitmek üzereyken ışıklar bir anda yandı. - When the movie was about to end, the lights suddenly turned on.

It means "while it was about to end." Structure: bit- (finish) + -mek (infinitive) + üzere (about to/on the verge of) + -yken (while/when). The subject of this non-finite clause is Film, so Film bitmek üzereyken = "while the film was about to end."

Why is there a "y" in "üzereyken"? Shouldn’t it be "üzereken"?

The temporal suffix is -ken ("while/when"). When a suffix that starts with a vowel attaches to a word ending in a vowel, Turkish inserts a buffer y. So:

üzere + -ken → üzereyken
evde + -ken → evdeyken
okulda + -ken → okuldayken

How is "bitmek üzereyken" different from "biterken" or "bitiyorken"? Could I also say "bitecekken"?

bitmek üzereyken: "just when it was about to finish" (strong sense of imminence).
bitiyorken: "while it was in the process of finishing" (less immediate).
biterken: "as it was finishing/nearing its end" (fine in everyday speech, slightly less pointed than -mek üzereyken).
bitecekken: also "just as it was about to finish" (very close in meaning to -mek üzereyken, often a bit more colloquial).

Is "üzere" a suffix or a separate word? Any other common use?

Üzere is a postposition written separately but tightly linked to a verb’s infinitive. Two common uses:

Imminence: Yağmur yağmak üzere. (It’s about to rain.)
Purpose: Toplantıya katılmak üzere Ankara’ya gittim. (I went to Ankara to attend the meeting.)

Could I say "Film bitmek üzereydi" instead?

Yes. Film bitmek üzereydi = "The film was about to end." To keep the original timing relation, you’d make two clauses: Film bitmek üzereydi; ışıklar bir anda yandı. The original with -yken packages the first clause as a time background.

Does "yandı" mean "burned"? How does it mean "turned on"?

Yanmak primarily means "to burn/be on fire," but with lights/devices it also means "to be on/to light up." Context decides:

Ev yandı. (The house burned.)
Işıklar yandı. (The lights came on.) Morphology: yan-dı = root yan-
- simple past -dı.

What’s the difference between "yanmak" and "yakmak"? How do I mention who turned on the lights?

yanmak: intransitive ("to burn; to be on"): Işıklar yandı.
yakmak: transitive ("to burn; to turn on [a light/fire]"): Işıkları bir anda yaktılar. (They turned on the lights all of a sudden.) Colloquially, açmak ("to turn on") is also used with lights: Işıkları açtılar.

Why is the verb singular "yandı" even though "ışıklar" is plural? Is "yandılar" wrong?

In 3rd person, verbs don’t have to mark plural. With inanimate plural subjects, the verb is typically singular: Işıklar yandı. Işıklar yandılar is generally avoided unless you’re talking about animate subjects or you want strong stylistic emphasis.

What exactly does "bir anda" mean? How is it different from "birden", "birdenbire", "aniden", or "hemen"?

bir anda: "all of a sudden / in an instant."
birden / birdenbire: "all of a sudden" (colloquial; birdenbire is more emphatic).
aniden: "suddenly" (more formal/neutral).
hemen: "immediately/at once" (timing, not necessarily "suddenly"). All but hemen fit the original sense of "suddenly."

Where can "bir anda" go in the sentence? Does placement change the meaning?

Common options:

Işıklar bir anda yandı. (neutral order)
Bir anda ışıklar yandı. (fronted for emphasis on suddenness)
Işıklar yandı bir anda. (afterthought/emphasis in speech) Focus tends to sit right before the verb, so bir anda yandı highlights the suddenness.

Why is there no "the" before "ışıklar"? How do we know it’s "the lights"?

Turkish has no articles. A bare plural subject like ışıklar is often read as definite if context supplies a known set—here, the theater/cinema lights—so it corresponds to English "the lights." If you needed to stress indefiniteness, you’d rely on context or use something like bazı ışıklar ("some lights").

Could I use "-ince" (when), like "Film bitince"?

That changes the timing. Film bitince ışıklar yandı means "After the film finished, the lights came on." The original means the lights came on right before it finished—while it was about to end.

How do I negate this or ask a yes–no question?

Negative: Işıklar bir anda yanmadı. (The lights didn’t come on all of a sudden.)
Question: Işıklar bir anda yandı mı? (Did the lights come on all of a sudden?) The question particle mi/mı/mu/mü is separate and vowel-harmonizes: yandı mı.

Can I say "lambalar" instead of "ışıklar"?

Yes. Işıklar emphasizes the light (illumination), while lambalar points to the fixtures/bulbs. In this context, Işıklar yandı is the most idiomatic, but Lambalar yandı is also understandable.

Are there other natural ways to say "right before it ended"?

Yes, for variety:

Filmin bitmesine az kala ışıklar yandı. (The lights came on with just a little time left before it ended.)
Filmin bitmesine yakın ışıklar yandı. (The lights came on near the end.)
You can also intensify with tam: Tam film bitmek üzereyken ışıklar… (Exactly as the film was about to end…)