Die Rücksendung kommt morgen.

Because Rücksendung is a feminine noun in German, so its nominative singular article is die.

In this sentence, die Rücksendung is the subject, so it appears in the nominative case:

• die Rücksendung = the return shipment / the return

If you changed the case, the article could change:

• nominative: die Rücksendung
• accusative: die Rücksendung
• dative: der Rücksendung
• genitive: der Rücksendung

So here, die is used because the noun is feminine and it is the subject of the sentence.

Rücksendung usually means something like:

• return shipment
• returned item
• return delivery
• sometimes simply return

It is built from:

• rück- = back / return
• Sendung = shipment, dispatch, consignment

So the basic idea is something being sent back.

Depending on context, it could refer to:

• the process of sending something back
• the parcel/item that is being returned

In a shopping or delivery context, die Rücksendung kommt morgen most naturally means that the returned shipment will arrive tomorrow.

Yes, kommt literally means comes, but in German, kommen is very often used where English would say arrive.

So German can say:

• Die Rücksendung kommt morgen.

Where natural English might be:

• The return arrives tomorrow.
• The returned package will arrive tomorrow.

This is completely normal German. German often uses kommen for people, packages, letters, buses, trains, and other things that are on the way to a destination.

Kommt is the 3rd person singular present tense of kommen.

The subject is die Rücksendung, which is singular, so the verb must also be singular.

Conjugation of kommen in the present:

• ich komme
• du kommst
• er/sie/es kommt
• wir kommen
• ihr kommt
• sie/Sie kommen

Since Rücksendung is a singular noun, German uses kommt.

German often uses the present tense to talk about the future when there is a clear time word such as morgen.

So:

• Die Rücksendung kommt morgen.

is a very normal way to say:

• The return is coming tomorrow
• The return arrives tomorrow
• The return will arrive tomorrow

German does have a future tense with werden, but it is not necessary here:

• Die Rücksendung wird morgen kommen.

That version is grammatical, but the simple present is usually more natural in everyday speech.

Here, morgen means tomorrow.

A very important clue is capitalization:

• morgen = tomorrow / tomorrow morning depending on context, but usually tomorrow as an adverb
• der Morgen = the morning

So:

• Die Rücksendung kommt morgen. = The return arrives tomorrow.
• Die Rücksendung kommt am Morgen. = The return arrives in the morning.

Lowercase morgen in this sentence is an adverb, not a noun.

Because morgen is being used as an adverb, not as a noun.

Adverbs do not take articles:

• heute = today
• morgen = tomorrow
• bald = soon

So:

• Die Rücksendung kommt morgen.

But if you use the noun Morgen meaning morning, then you need an article or some other determiner:

• am Morgen = in the morning
• jeden Morgen = every morning

German main clauses usually follow the verb-second rule, often called V2.

In this sentence:

• Die Rücksendung = first element
• kommt = second element
• morgen = later in the sentence

So the pattern is:

• Subject + verb + time expression

That gives:

• Die Rücksendung kommt morgen.

You can also move another element to the first position, but the verb still stays second:

• Morgen kommt die Rücksendung.

Both are correct. The difference is mainly emphasis.

Yes, absolutely.

Both of these are correct:

• Die Rücksendung kommt morgen.
• Morgen kommt die Rücksendung.

The second version puts more emphasis on morgen.

This is a very common feature of German word order:

• the verb still stays in second position
• whatever comes first is often what the speaker wants to highlight

So:

• Die Rücksendung kommt morgen. = neutral
• Morgen kommt die Rücksendung. = emphasis on tomorrow

A helpful approximate pronunciation is:

RUEK-zen-doong

A bit more carefully:

• Rück sounds roughly like rük with the German ü
• sen sounds like zen because the s here is pronounced like z
• dung sounds like doong

A few pronunciation notes:

• ü does not exist in standard English. It is made with your tongue in an ee position but with rounded lips.
• ck gives a short vowel before it, so Rück has a short ü.
• The stress is usually on the first part: RÜCK-sendung.

It is neutral and works in both formal and informal contexts.

Nothing in the sentence is especially formal or casual:

• Die Rücksendung is standard vocabulary
• kommt morgen is normal everyday German

You could see this in:

• customer service messages
• delivery updates
• ordinary conversation

So it is a very natural standard sentence.