Learn Spanish (Latin America): Apenas usé el extintor, todos se calmaron. - As soon as I used the fire extinguisher, everyone calmed down.

Because the action is a completed fact in the past. When apenas (as soon as) refers to an action that already happened or happens habitually, it takes the indicative. If you were talking about an anticipated event in the future (like "As soon as I use the fire extinguisher..."), you would need the present subjunctive: apenas use.

I thought apenas meant "barely." Why does it mean "as soon as" here?

Apenas can definitely mean "barely" or "hardly" (as in apenas tengo tiempo for "I barely have time"). However, when it is used to introduce a time clause right before a verb, it translates to "as soon as" or "the moment that." In this context, it functions just like en cuanto.

Why do we need the se in se calmaron?

The verb calmar on its own means "to calm someone else down" (for example, calming a crying baby or an angry customer). Because the people in this sentence calmed themselves down, you have to use the reflexive form calmarse.

Could I use the imperfect tense here instead of the preterite (usé, calmaron)?

No, you need the preterite here because these are viewed as completed, single events in a sequence. You performed one quick, finished action (using the extinguisher), which immediately triggered the next finished event (everyone calming down). The imperfect would only be used if you were describing a repeated habit in the past, like "As soon as I used to use the extinguisher..."