Learn Hardcore Turkish: Ben sınavdayken lütfen kimse kapıyı çalmasın. - While I am in the exam, please no one knock on the door.

Break it down: sınav + da + (i)ken → sınavdayken.
-DA is the locative suffix “at/in,” and (i)ken means “while/when.”
Because sınavda ends in a vowel, Turkish inserts a buffer y before -ken: sınavda iken → sınavdayken.
Similar examples: evdeyken (while at home), işteyken (while at work), Türkiye’deyken (while in Turkey).

Do I need to say ben, or can I just say Sınavdayken lütfen kimse…?

You can drop ben if the context already makes it clear who is in the exam.
Sınavdayken itself doesn’t mark the person; without ben it could refer to I/you/he/she/we/they depending on context.
Ben makes it explicit: it’s “while I am in the exam.”

What mood/tense is çalmasın?

It’s the 3rd person singular negative of the optative/imperative: “let him/her/it not …,” often rendered as “should not.”
Morphology: çal- (knock/ring) + -ma (negation) + -sın (3rd person optative) → çalmasın.
Affirmative counterpart: çalsın (“let him/her knock”).

Why is it kapıyı (accusative) and not kapıya?

Kapıyı çalmak is a fixed transitive construction in Turkish: the door is the direct object, so it takes accusative.
Kapıya çalmak is not used.
You can also say zili çalmak (“ring the bell”).

Why does kapı become kapıyı?

Accusative is -(y)I with vowel harmony.
kapı + (y)I → kapıyı: the buffer y appears because kapı ends in a vowel; the vowel is ı by harmony with the last vowel ı.

Does çalmak also mean “to steal”? Isn’t kapıyı çalmasın ambiguous?

Yes, çalmak can mean “to steal,” “to knock,” “to ring (a bell),” or “to play (an instrument).”
The collocation kapıyı çalmak specifically means “to knock on the door” (and by extension “ring the doorbell” in everyday speech).
Context and the object disambiguate: with kapıyı, speakers understand “knock,” not “steal.” To clearly say “ring the bell,” use zili çalmak.

Could I say çalmayın instead of çalmasın?

Çalmayın = 2nd person plural/polite negative imperative: “Please don’t (you) knock.”
Çalmasın = 3rd person: “Let no one knock.”
Choose based on who you’re addressing: specific listeners (çalmayın) vs. an indefinite “anyone” (çalmasın).

Should the verb be singular or plural after kimse? Is çalmasınlar OK?

Standard is singular: Kimse … çalmasın.
Colloquially, çalmasınlar can appear to emphasize a group, but singular is the neutral, preferred form.

What exactly does kimse mean here?

Kimse in negative contexts means “nobody/no one.”
In questions or conditionals, it can mean “anybody/anyone” (e.g., Kimse geldi mi? “Did anyone come?”).
With a negative verb, as in kimse … çalmasın, it means “no one should …”.

Where does lütfen usually go?

Most common and natural at the beginning: Lütfen, ben sınavdayken kimse kapıyı çalmasın.
Also fine before the verb phrase: Ben sınavdayken lütfen kimse kapıyı çalmasın.
Less common but possible at the end for softer tone: … kapıyı çalmasın lütfen.

Should there be a comma after Ben sınavdayken?

Optional. Many writers put a comma when the temporal clause comes first: Ben sınavdayken, lütfen…
Omitting it is acceptable in informal writing.

Are there more formal or alternative ways to say “while I’m in the exam”?

Sınav sırasında / sınav esnasında = “during the exam” (more formal).
Sınavda iken is a spaced, more formal version of sınavdayken.
Don’t say sınavdayımken; -ken does not attach to personal-predicate forms like -yım.

How would I say “while I was in the exam, nobody knocked”?

Can I strengthen “no one” with hiç?

Yes: Lütfen hiç kimse kapıyı çalmasın = “Please absolutely no one should knock.”
Hiç kimse is emphatic; it still requires a negative verb.

Does kapıyı çalmak mean knock or ring?

In many contexts it covers both “knock” and “ring (the doorbell).”
If you want to be specific: kapıyı tıklatmak (knock gently), zili çalmak (ring the bell).