Learn Hardcore French: J’ai retrouvé mes clés dans mon sac.

trouver = to find/discover (no prior possession implied): J’ai trouvé une pièce. (I found a coin.)
retrouver = to find again/recover something you had or misplaced; also “to meet up again” with people: J’ai retrouvé mes clés. (I found my keys again / I recovered my keys.)

Why is the passé composé used here?

Because it’s a single, completed event in the past. French passé composé often corresponds to both English simple past (“I found”) and present perfect (“I’ve found”), depending on context.

Why is it ai (from avoir) and not suis (from être)?

Most transitive verbs (those that take a direct object) form the passé composé with avoir. Retrouver is transitive, so: J’ai retrouvé… Not Je suis retrouvé… (that would be incorrect here).

How would the sentence change if I replace “mes clés” with a pronoun?

Use the direct object pronoun before the auxiliary: Je les ai retrouvées dans mon sac.
Agreement: with avoir, the past participle agrees with a preceding direct object. Les refers to clés (feminine plural), so add -es: retrouvées.
Negation: Je ne les ai pas retrouvées.

Why is it mes clés but mon sac?

Possessive adjectives agree with the thing owned:

mes clés: clés is plural → mes (plural for either gender).
mon sac: sac is masculine singular → mon. Compare: ma clé (one key, feminine), mes clés (keys, plural).

Why use dans here? Could I use en or à?

Use dans for something physically inside a container/place: dans mon sac.

en is not used for “in a specific bag”; it’s used with countries, seasons, materials, transport modes, etc. (e.g., en France, en été, en bus).
à typically means “at/to/in” with cities or indicates direction/indirect objects; not correct for “in my bag.”

Could I say dans le sac instead of dans mon sac?

Yes, if you mean “in the bag” previously mentioned or understood from context. Dans mon sac emphasizes it’s your bag. Both are grammatically correct; choose based on the meaning you want.

How do you pronounce the sentence?

Approximation: “zhay rə-troo-VAY may KLAY dahn mon sak.”
Notes:

J’ai ≈ “zhay”
retrouvé final é sounds like “ay”
clés final s silent
dans final s silent; the vowel is nasal (“dahn”)
mon has a nasal “on” sound (“mohn”)
No liaison in mes clés (the c/k sound blocks it)

How do I make it negative?

Wrap the auxiliary with ne…pas: Je n’ai pas retrouvé mes clés dans mon sac.
(Elide to n’ before the vowel sound in ai.)

How do I ask “Where did you find your keys?” using this verb?

Inversion (with tu): Où as-tu retrouvé tes clés ?
Est-ce que: Où est-ce que tu as retrouvé tes clés ?
With vous: Où avez-vous retrouvé vos clés ?
Very informal: Tu as retrouvé tes clés où ?

Is it clé or clef?

Both are correct. Clé/clés is now more common in modern usage. Clef survives in some set phrases (e.g., clef de sol) and stylistic choices. Pronunciation is the same.

Why is there an acute accent in retrouvé and clé(s)?

retrouvé: past participle of an -er verb ends in -é (distinct from the infinitive retrouver).
clé: the accent marks the vowel quality; standard modern spelling uses the accent.

Can I say J’ai trouvé mes clés instead? Does it change the meaning?

Yes. J’ai trouvé is always acceptable. J’ai retrouvé adds the nuance that you recovered them (you had them before and misplaced them). For lost items, retrouver often sounds more idiomatic.

Can I move dans mon sac to another position?

Yes, especially for emphasis:

Fronting: Dans mon sac, j’ai retrouvé mes clés.
With the locative pronoun: J’y ai retrouvé mes clés. (Here y = “there,” i.e., in my bag.)
Neutral, most common order is the original: J’ai retrouvé mes clés dans mon sac.