Learn Hardcore German: Im Büro kümmert sich die Verantwortliche um die E-Mails, der Verantwortliche plant die Treffen. - At the office the woman in charge takes care of the emails, the man in charge plans the meetings.

You use in dem (→ im) here because Büro is neuter (das Büro) and you’re talking about location (answering “where?”), which takes the dative case in German: in dem Büro → im Büro.

Why is Büro in the dative case after in?

The preposition in can take either accusative or dative:

Accusative: movement into something (wohin? – where to?)
Dative: position in/at something (wo? – where?)

Here, Im Büro describes where the actions happen (no movement), so Büro must be in the dative: in dem Büro → im Büro.

Why is the word order kümmert sich die Verantwortliche instead of die Verantwortliche kümmert sich?

German main clauses follow the “verb-second” rule: the finite verb must be the second element in the sentence.
The first element here is the prepositional phrase Im Büro. Therefore, the verb kümmert has to come next, in second position. The rest (sich die Verantwortliche um die E-Mails) follows after the verb.

So both are possible:

Die Verantwortliche kümmert sich im Büro um die E-Mails. (subject first)
Im Büro kümmert sich die Verantwortliche um die E-Mails. (place first, but verb still second)

Why do we need sich with kümmern? Is kümmern always reflexive?

In the meaning “to take care of / to look after”, kümmern is used reflexively: sich um etwas kümmern. The sich is part of the verb pattern and can’t be omitted in this sense.

sich kümmern um + Akkusativ = to take care of something/someone
In the sentence, sich refers back to die Verantwortliche (the one who is taking care).

Why is it um die E-Mails and not für die E-Mails or über die E-Mails?

The expression is fixed: sich um etwas kümmern (always um + accusative).

um here doesn’t mean “around”; it’s just part of this verb phrase.
für die E-Mails kümmern or über die E-Mails kümmern is incorrect.

You must say:

sich um die E-Mails kümmern = to deal with / handle the emails.

Which case is die E-Mails in, and why?

die E-Mails is in the accusative plural.
The preposition um (in this meaning) always governs the accusative case. So:

um + Akkusativ → um die E-Mails

In the plural, die is the same for nominative and accusative, but here we know it’s accusative because of um.

Why are Verantwortliche and Verantwortliche capitalized, and what kind of word is this?

Verantwortliche is originally an adjective (verantwortlich = responsible), but here it is used as a noun meaning “the person in charge”. Adjectives used as nouns must be capitalized in German:

die Verantwortliche = the (female) person in charge
der Verantwortliche = the (male) person in charge

So Verantwortliche is a noun formed from an adjective (an “adjectival noun”), which is why it’s capitalized.

Why is it die Verantwortliche in the first clause but der Verantwortliche in the second? Does that mean two different people?

Yes, grammatically it refers to two different people:

die Verantwortliche = feminine (a woman)
der Verantwortliche = masculine (a man)

German articles (der / die / das) mark gender. So the sentence suggests that a woman in charge handles the emails, while a (possibly different) man in charge plans the meetings.

How are the endings in die Verantwortliche and der Verantwortliche determined?

When an adjective is turned into a noun with an article, it takes adjective endings:

Masculine nominative singular with der → der Verantwortliche (-e)
Feminine nominative singular with die → die Verantwortliche (-e)

Both end in -e here because of the case (nominative) and the article; the gender is shown mainly by der/die, not by the ending.

Why is there only a comma between the two parts, and is it correct to join two main clauses with just a comma?

Yes, in German it is correct to connect two main clauses with just a comma (without und, aber, etc.):

Im Büro kümmert sich die Verantwortliche um die E-Mails, der Verantwortliche plant die Treffen.

This is a standard way to join related independent clauses. You could also write two separate sentences or connect them with und, but the comma alone is grammatically acceptable in German.

What does Treffen mean here, and how can I tell if Treffen is singular or plural?

Treffen as a noun means meeting / get-together.
Its forms are:

Singular: das Treffen (one meeting)
Plural: die Treffen (several meetings; same word form)

In the sentence, die Treffen must be plural because:

The article die with a capitalized noun and no adjective strongly suggests plural neuter here.
Contextually, “plans the meetings” makes sense.
So die Treffen = the meetings.

Could I say Im Büro kümmert die Verantwortliche sich um die E-Mails instead? Where can sich go?

Yes, Im Büro kümmert die Verantwortliche sich um die E-Mails is also correct.
In German, unstressed pronouns like sich usually go early in the “middle field” (the part between the conjugated verb and the sentence-final elements), but there is some flexibility:

Im Büro kümmert sich die Verantwortliche um die E-Mails.
Im Büro kümmert die Verantwortliche sich um die E-Mails.

Both are grammatically fine; the original version is more common/neutral.

What is the difference in meaning between sich um etwas kümmern and etwas planen in this sentence?

sich um die E-Mails kümmern = to take care of the emails, handle them, deal with them, respond to them, organize them, etc.
die Treffen planen = to plan the meetings (decide time, place, agenda, participants, etc.).

So the sentence contrasts two different responsibilities: one person is handling ongoing tasks (emails), and the other is organizing future events (meetings).