Learn German: Falls ein Notfall passiert, brauchen wir Bargeld. - In case an emergency happens, we need cash.

Falls is a subordinating conjunction, so it kicks the verb of its own clause (passiert) to the very end. Then, because that entire Falls clause takes up the first position of the main sentence, the main verb (brauchen) has to come immediately after the comma to stay in the standard Position 2.

Could I use wenn instead of falls here?

Yes! Both can be used to set up a condition. However, falls specifically means "in case" or "in the event that." It makes the emergency sound like a mere possibility, whereas wenn (if or when) can sometimes imply you actually expect it to happen.

What case is ein Notfall in this sentence?

It is in the nominative case because it is the subject of the first clause, since it is the thing that is happening. Because Notfall is a masculine noun (der Notfall), it takes the basic masculine nominative article ein.

What does the Bar- in Bargeld mean?

The prefix bar- comes from an old root meaning "bare" or "uncovered" (just like in barfuß, meaning "barefoot"). So Bargeld essentially means "bare money", meaning actual, physical coins and bills rather than digital currency.