Die Spedition hat die Möbel geliefert, aber ein Karton ist noch unterwegs.

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.

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).

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.

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.

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

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.

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).

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

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.

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).

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 ...)