Learn Hardcore Turkish: Sinemada bizden başka kimse yok. - There is no one else in the cinema besides us.

Because başka in the meaning of except/other than is a postposition that requires the ablative on the thing being excluded. Pattern: X’den başka Y.

Senden başka kimse yok. = There’s no one except you.
Bu şehirden başka yer yok. = There’s no place other than this city.

Why use yok instead of değil?

var/yok state existence or non-existence: kimse yok = nobody exists.
değil negates identity/quality: Ali sinemada değil = Ali is not at the cinema. So for “there isn’t,” Turkish uses yok, not değil.

Is kimse singular or plural? Why not kimseler?

Kimse is grammatically singular and is the normal word for “no one/anyone.” Kimseler exists but is rare/poetic and not needed in everyday speech. Use kimse here.

Can I say Sinemada bizden başkası yok? Is there any difference from ... başka kimse yok?

Yes, both are correct and very natural.

bizden başka kimse yok = apart from us, nobody (else) exists.
bizden başkası yok = apart from us, there isn’t any other (person). Meaning and tone are virtually the same; the first explicitly says “nobody,” the second says “no other (one).”

Do I need hiç (as in hiç kimse yok)?

No. Kimse yok already means “no one.” Adding hiç intensifies/emphasizes:

Sinemada bizden başka kimse yok. = There’s no one else.
Sinemada bizden başka hiç kimse yok. = There’s absolutely no one else at all.

Can I change the word order?

Yes; word order is flexible for emphasis. Common variants:

Sinemada bizden başka kimse yok. (neutral/default)
Bizden başka sinemada kimse yok. (slight emphasis on “apart from us”)
Bizden başka kimse sinemada yok. (emphasis on the location) Keep yok at the end; that’s the usual place for the predicate.

How do I ask “Is there anyone else in the cinema besides us?”

Neutral information-seeking: Sinemada bizden başka kimse var mı?
Expecting/assuming “no”: Sinemada bizden başka kimse yok mu?

How do I say it in the past or future?

Past: Sinemada bizden başka kimse yoktu. = There wasn’t anyone else.
Future: Sinemada bizden başka kimse olmayacak. = There won’t be anyone else. (For future, use olmayacak rather than trying to inflect yok.)

What’s the difference between sinemada and sinema da?

sinemada (one word) = “in the cinema” (locative suffix -da).
sinema da (two words) = “the cinema, too/also” (da/de as the additive particle). So Sinemada starts the sentence you asked about; Sinema da would mean “The cinema also …”

Why is it -da (not -de or -ta) in sinemada?

Locative suffix -DA follows vowel harmony and consonant voicing:

Back vowel → -da; front vowel → -de.
After a voiceless consonant, the suffix also devoices: -ta/-te. Examples:
sinemada (back vowel a → -da)
şehirde (front vowel e → -de)
kitapta (voiceless p → -ta)

Can I replace bizden başka with other expressions like bizim dışımızda or biz hariç?

Yes—natural equivalents:

Bizim dışımızda = outside of us/apart from us (a bit more formal/literal).
Biz hariç = except us. Examples:
Sinemada bizim dışımızda kimse yok.
Sinemada biz hariç kimse yok.

Could I say Sinemada bizden başka biri yok?

Yes. biri/birisi = “someone.” With negation it means “no someone,” i.e., no one.

Sinemada bizden başka biri (birisi) yok. = There isn’t anyone else besides us. It’s fine, though kimse is the most common choice with yok.

What question would this sentence naturally answer?

Sinemada kim var? = Who is in the cinema?
Sinemada bizden başka kim var? = Who else is there besides us? Answer: Bizden başka kimse yok.

Do I need an apostrophe, like Sinema’da?

No. Apostrophes are for proper names: İstanbul’da, Ankara’da. Common nouns take suffixes without apostrophes: sinemada.

Can kimse appear in positive sentences?

In negative: kimse = nobody. Example: Kimse yok.
In questions/conditionals: kimse = anyone. Example: Kimse var mı? (Is anyone there?)
In plain affirmative statements, kimse meaning “anyone” is uncommon; you’d use biri (someone) or herkes (everyone) instead.

How else can I express the same idea with sadece (only)?

Sinemada sadece biz varız. = Only we are in the cinema. This uses varız (we exist/are there) instead of kimse yok, but the meaning matches.