Learn Hardcore German: Die Spedition hat die Möbel geliefert, aber ein Karton ist noch unterwegs. - The moving company delivered the furniture, but one box is still on its way.

Spedition is a feminine noun, so it takes die in the nominative: die Spedition.
Unfortunately, gender is often something you learn with the noun, but there are hints:

Nouns ending in -tion are almost always feminine in German (like die Information, die Station, die Situation). So die Spedition fits that pattern.

What exactly does Spedition mean? Is it the same as a shipping company?

Yes, die Spedition is typically a freight forwarding / logistics / shipping company—a business that organizes and carries out transport of goods (often by truck, but also other modes). In everyday use, it can be the delivery/logistics company handling your shipment.

Why is it hat ... geliefert instead of a simple past form?

German often uses the Perfekt tense in spoken and informal contexts to talk about completed past actions.
Here, hat geliefert = Perfekt of liefern.

liefern is a regular verb: liefern – lieferte – hat geliefert You can also say Die Spedition lieferte die Möbel, but that sounds more written/formal or narrative (depending on region and style).

Why is the verb split: hat near the beginning and geliefert at the end?

That’s the normal structure for the Perfekt in a main clause:

finite auxiliary verb (hat/ist) goes in position 2
past participle (geliefert) goes to the end So: Die Spedition hat die Möbel geliefert.

Why is it die Möbel? Is there a singular form?

Möbel is commonly treated as a plural-only noun in modern German when you mean furniture in general (similar to English furniture being uncountable).
If you want a single piece, you’d usually say something like:

ein Möbelstück (a piece of furniture)
or name the item: ein Stuhl, ein Tisch, ein Schrank, etc.

What case is die Möbel here, and how can I tell?

die Möbel is accusative because it’s the direct object of liefern (to deliver something).
In this particular sentence, die Möbel looks the same in nominative and accusative because die + plural doesn’t change. But functionally it’s the object:

Subject: die Spedition
Verb: hat geliefert
Object: die Möbel

Why is there a comma before aber?

Because aber connects two independent main clauses here: 1) Die Spedition hat die Möbel geliefert
2) ein Karton ist noch unterwegs
German normally puts a comma between two main clauses joined by aber, denn, sondern, etc.

Why does the second clause use ist (not hat)?

The second clause isn’t Perfekt; it’s just the present tense of sein: ist.
Here unterwegs describes a state/condition (being on the way), not a completed action. So German naturally says:

Ein Karton ist unterwegs. = A box is in transit / on the way.

What part of speech is unterwegs? Adjective or adverb, and why isn’t there an ending?

unterwegs functions like an adverb/predicative expression meaning on the way / in transit.
It typically appears:

after sein: Er ist unterwegs. It doesn’t take adjective endings here because it’s not used directly before a noun (you wouldn’t normally say ein unterwegs-er Karton).

What does noch add here, and where can it go in the sentence?

noch here means still / yet: the box hasn’t arrived so far.
Position is flexible, but it usually sits near what it modifies:

ein Karton ist noch unterwegs (very natural) You could also emphasize it:
ein Karton ist immer noch unterwegs = still (even now), more emphatic

Why does it say ein Karton and not der Karton?

ein Karton introduces it as one (unspecified) box—often meaning one of the boxes or a remaining box.
You’d use der Karton if it’s already specific/known in the conversation, like:

Der Karton ist noch unterwegs = that particular box we’re talking about is still in transit.

Is Karton masculine, and how does its case work here?

Yes, Karton is masculine: der Karton.
In the sentence it’s the subject of the second clause, so it’s nominative:

ein Karton (nominative masculine indefinite article)
If it were an object, it could change (e.g., Ich warte auf einen Karton in accusative after auf with motion/goal in some contexts).

Can I replace aber with something else like doch or jedoch?

Often, yes, with slight style differences:

..., aber ein Karton ist noch unterwegs. (neutral, very common)
..., doch ein Karton ist noch unterwegs. (a bit more contrastive; can sound slightly more formal/literary)
..., jedoch ist ein Karton noch unterwegs. (more formal; often pushes inversion: jedoch ist ...)