Learn Hardcore Dutch: In dat geval bel je de conciërge. - In that case, you call the caretaker.

in is a preposition meaning “in.”
dat is a demonstrative pronoun used with het-words (neuter nouns) meaning “that.”
geval is a neuter noun meaning “case” or “situation.”
Put together, in dat geval translates as “in that case” or “in that situation.”

Why is it dat and not die or deze in in dat geval?

Dutch demonstratives agree with the gender of the noun:

deze/ die go with de-words (common gender).
dit/ dat go with het-words (neuter).
Since geval is a neuter noun (het geval), you must use dat.

Why does the verb bel come before the subject pronoun je? Shouldn’t it be je belt?

Dutch follows the V2 (verb-second) rule. When an adverbial phrase (in dat geval) starts the sentence, the finite verb moves into the second position, pushing the subject after it.

Correct inversion: In dat geval bel je de conciërge.
Normal word order (subject first): Je belt de conciërge in dat geval.

What exactly is a conciërge, and how do you pronounce it?

A conciërge is a caretaker or janitor, often the person responsible for building maintenance or reception duties.
Pronunciation: /kɔn.siˈɛr.ɣə/

Why does conciërge have a diaeresis (two dots) on the ë?

The diaeresis indicates that the e is pronounced as a separate syllable from the preceding i, so you get four syllables: con-ci-èr-ge. Without it, you might read ie as one sound.

How would you make this sentence more formal by using u instead of je?

You switch je bel to u belt (formal pronoun + verb form):
In dat geval belt u de conciërge.

What if you want to express obligation: “In that case, you must call the concierge”?

Insert the modal verb moeten. Because je is the subject, you use moet and move bellen (the infinitive) to the end:
In dat geval moet je de conciërge bellen.

Can you rephrase the sentence as a direct command (imperative)?

Yes. Put the verb in front and drop the subject pronoun:
Bel in dat geval de conciërge.