Learn Hardcore German: In der Notaufnahme ist die Wartezeit am Abend oft länger als am Morgen. - In the emergency room, the waiting time in the evening is often longer than in the morning.

Dative = location (where?): In der Notaufnahme = in the ER / in the emergency department (already there).
Accusative = direction (where to?): In die Notaufnahme = into the ER (moving there).

Here the sentence describes where the waiting time is longer, so it uses dative.

Why does der appear in in der Notaufnahme—isn’t Notaufnahme feminine?

It is feminine: die Notaufnahme (nominative).
But after in with a location meaning, you use dative, and the feminine dative singular article is der:

nominative: die Notaufnahme
dative: in der Notaufnahme

What role does die Wartezeit play in the sentence?

die Wartezeit is the subject (the thing being described). The basic structure is:

die Wartezeit (subject) + ist (verb) + länger (predicate/adjective)

Even though In der Notaufnahme is placed first, the subject is still die Wartezeit.

Why is the verb ist so early—why not In der Notaufnahme die Wartezeit ist …?

German main clauses follow the verb-second (V2) rule: the conjugated verb is in position 2.
If you put In der Notaufnahme in position 1, the verb ist must come next, and the subject moves after it:

In der Notaufnahme
ist
die Wartezeit …

So In der Notaufnahme ist die Wartezeit … is standard word order.

What does am Abend / am Morgen literally mean, and what case is that?

am is a contraction of an dem:

am Abend = an dem Abend
am Morgen = an dem Morgen

These are dative because an here is used in a “time-at” sense (similar to on/at in English, but German uses dative in this fixed expression).

Why is it länger als and not something like mehr lang als?

German typically forms comparatives with -er:

lang → länger (long → longer)

To compare, German uses als:

länger als am Morgen = longer than in the morning

mehr is used for comparisons with many nouns/quantities (e.g., mehr Zeit = more time), but with the adjective lang, the natural comparative is länger.

Why doesn’t länger have an adjective ending (like längere or längeren)?

Because länger is used predicatively (after sein), and predicative adjectives in German do not take endings:

Die Wartezeit ist länger. (no ending)

Adjective endings appear when the adjective is attributive (in front of a noun):

eine längere Wartezeit = a longer waiting time

Where does oft go in German word order? Could it go somewhere else?

oft is an adverb of frequency and commonly sits in the “middle field,” often before the adjective/predicate:

… ist die Wartezeit am Abend oft länger …

Other placements are possible for emphasis, but can sound marked:

Oft ist die Wartezeit am Abend länger … (emphasizes often)
… ist die Wartezeit am Abend länger … (neutral but removes the “often” nuance)

Is am Abend the same as abends? And am Morgen the same as morgens?

They’re similar but not identical in feel:

am Abend / am Morgen = in the evening / in the morning (more concrete, “at that time of day”)
abends / morgens = in the evenings / in the mornings (more general, habitual)

Your sentence uses am Abend / am Morgen, which fits well with a general observation too.

Why are words like Notaufnahme, Wartezeit, Abend, and Morgen capitalized?

Because they are nouns, and all German nouns are capitalized:

die Notaufnahme
die Wartezeit
der Abend
der Morgen

This is a consistent spelling rule in German.

What exactly does Notaufnahme refer to—does it mean the same as ER?

Yes, die Notaufnahme is the usual German term for a hospital’s emergency department—very close to ER (US) or A&E (UK). In context, it means the place where patients are received and treated urgently.

How would you pronounce the key words in this sentence?

Approximate pronunciations (English-friendly):

Notaufnahme: NOHT-owf-nah-muh (stress usually on Not-)
Wartezeit: VAR-tuh-tsайт (with Zeit like German tsайт)
länger: LENG-er (umlaut ä is like “e” in bed, but longer)
Morgen: MOR-gen (hard g sound)