Learn Turkish: Bina çöker çökmez enkaz altında kalanlar için yardım geldi. - Help arrived for those who remained under the debris as soon as the building collapsed.

This uses the structure you are learning now: the aorist affirmative ending (-er) immediately followed by the aorist negative ending (-mez) on the same verb root (çök-). Together, -Ir -mAz creates the meaning of 'as soon as' or 'the moment that' the action happens. Even though the main verb geldi is in the past tense, this structure doesn't change to a past tense form itself; it just takes its time reference from the main verb.

What is the grammar behind the word kalanlar?

It is formed from the verb kalmak (to stay or remain). We add the subject participle -an to make kalan (the one who remains). By adding the plural marker -lar, it functions as a noun meaning 'those who remained' or 'the ones who stayed.'

Why does altında have an -n- in the middle?

To say 'under the debris,' Turkish uses the noun alt (bottom). We make a compound: enkaz altı (the debris' bottom). To say 'at' or 'in' that location, we add the locative suffix -da. The -n- is simply a buffer letter that is always required when adding a spatial suffix like -da to a possessive suffix like the -ı in altı.

What exactly does için apply to in this sentence?