You're right, she does it badly. You should not hear any "s" sound : "SETIL" (not SESTIL")

Yes, more or less also the equivalent of "what's up?" or "what's wrong?" or "what's going on?"...

"se passer" is a reflexive verb meaning "happen/occur".

"il" in this sentence is impersonal, not "he".

I don't think it is more useful, because it is not correct to translate a question.

what you propose is "...that what happened here."

Yes, "qu'est ce qui est arrivé ici ?" or "qu'est ce qu'il est arrivé ici ?" are both correct and accepted.

I don't know if there is an easy way, other than learning by heart a few key sentences. But there are lots of parameters (single or compound verb; presence/absence of an object pronoun; formal or informal questions; binary or open questions; affirmative or negative...)

Basically, Direct or Indirect Object pronouns, and reflexive pronouns are placed before the verb or the auxiliary.

• je le donne (DO) - je te donne (IO) - je te le donne (IO+DO)
• je l'ai donné (DO) - je t'ai donné (IO) - je te l'ai donné (IO+DO

When the IO is 3rd person singular or plural, it is switched with the DO.

• je le donne (DO) - je leur donne (IO) - je le leur donne (DO+IO)
• je l'ai donné (DO) - je lui ai donné (IO) - je le lui ai donné (DO+IO)

If the sentence is negative, the first negative comes before the object/reflexive pronouns and the second negative comes after the verb/auxiliary.

• je ne le donne pas - je ne leur donne pas - je ne te donne pas - je ne le leur donne pas
• je ne l'ai pas donné - je ne leur ai pas donné - je ne te l'ai pas donné - je ne le leur ai pas donné