Learn Hardcore Turkish: İlk adımda tökezleyince koç ‘sakin ol’ dedi. - When he stumbled on the first step, the coach said “stay calm.”

It’s the temporal converb -ınca/-ince meaning “when/once/after (someone) V-ed.”
Form: tökezle-y-ince = tökezle- (to stumble) + buffer -y- (because the stem ends in a vowel) + -ince (vowel-harmony form of -ınca).
It sets the time/trigger for the main clause: “When (he/she) stumbled, … the coach said …”

Who is understood to be the subject of tökezleyince?

It’s the person who stumbled (contextually, the athlete), not the coach. Turkish often leaves the subject of such adverbial clauses implicit. If you want to make it explicit, you can say: Sporcu ilk adımda tökezleyince, koç … or O ilk adımda tökezleyince, koç …

Is there a comma missing after the initial clause?

Many writers put a comma after an initial -ince clause: İlk adımda tökezleyince, koç …. It’s optional with short, clear clauses; adding it improves readability but leaving it out isn’t wrong.

What exactly does ilk adımda mean, and what case is -da?

It means “at/on the first step” (literally) or “at the first stage/step” (figuratively). -da is the locative case (“in/at/on,” depending on context): adım (step) + -da (locative). Turkish has no articles, so definiteness (“the first step”) comes from context.

Does adım also mean “my name”? How do I tell them apart?

Yes, adım can be:

“step” (a standalone noun), or
“my name” (from ad “name” + possessive -ım). Context disambiguates: Adım Ali = “My name is Ali.” İlk adımda = “at the first step,” not “in my name.”

Why is the I dotted in İlk? And why isn’t koç capitalized?

Turkish distinguishes dotted and dotless I: lowercase i → uppercase İ; lowercase ı → uppercase I. Sentence-initial İlk must use İ. koç isn’t capitalized because it’s a common noun (“the coach”). It would be capitalized if it were a nickname or part of a proper name.

What does koç mean here—ram or coach?

Contextually it’s “coach” (the person training the athlete). koç can also mean “ram” (male sheep), but the sports context resolves the ambiguity.

Is the punctuation of the quoted words acceptable? Should there be commas or capitals?

Styles vary. Very common formal styling is: Koç, "Sakin ol," dedi. (capitalize the first word in the quote; comma inside the closing quote). Another accepted variant: Koç dedi: "Sakin ol." Your version with single quotes and no comma is seen informally; in careful writing, add the comma/capitalization.

Why dedi and not söyledi?

demek is preferred for direct quotations of exact words: Koç, "Sakin ol," dedi.
söylemek often takes a content clause or infinitive and is closer to “to tell (someone) that/to”: Koç, sakin olmamı söyledi (“The coach told me to be calm”). Both are possible, but they pattern differently.

How do I show who the coach was talking to?

Use the dative for the indirect object:

Koç bana/ona/Ali’ye "Sakin ol" dedi. = “The coach said ‘Calm down’ to me/him/her/Ali.”

What is the grammar of sakin ol? How do I make it negative or polite?

ol is the 2nd person singular imperative of olmak (“to be”): sakin ol = “be calm.”

Negative: sakin olma (“don’t be calm” → usually contextually “don’t panic” would be panik yapma).
Plural/formal: sakin olun.
To soften: add lütfen: Lütfen sakin olun.

Can I say sakin kal or sakinleş instead? What’s the nuance?

sakin ol = “be calm” (a general instruction).
sakin kal = “stay/remain calm” (don’t lose your calm).
sakinleş = “calm down” (become calm after being agitated).
All are natural; choose based on the situation.

What does tökezlemek mean exactly? Any near-synonyms?

tökezlemek = to trip/stumble (often because your foot catches). Near-synonyms: ayağı takılmak (“one’s foot gets caught”), sendelenmek (“to stagger”), yalpalamak (“to wobble”). düşmek is “to fall,” which may or may not happen after stumbling.

What’s the difference between -ince, -ken, and -diğinde?

-ınca/-ince: “when/once/after (event)”; often a single, completed trigger. Example: Tökezleyince koç uyardı (“Once he stumbled, the coach warned him”).
-ken: “while/when (ongoing/concurrent)”. Example: Koşarken tökezledi (“He stumbled while running”).
-dığında/-diğinde: “when (at the time that)”; a bit more explicit/formal and allows clear subjects via possessive: O tökezlediğinde koç uyardı.

Can I switch the clause order?

Yes. For example: Koç, "Sakin ol," dedi (o) ilk adımda tökezleyince. or more clearly: (Sporcu) ilk adımda tökezleyince, koç "Sakin ol" dedi. Turkish is flexible, but keep it unambiguous.

Why is it adımda, not adımta? How do the locative variants work?

The locative has four allomorphs by vowel and consonant harmony: -da/-de/-ta/-te.

Back vs front vowel: a, ı, o, u → -da; e, i, ö, ü → -de.
After voiceless consonants (p, ç, t, k, f, h, s, ş), use t; otherwise d.
adım ends with a voiced consonant (m) and has a back vowel (ı), so it takes -da → adımda.