Der Rückgabetermin ist morgen.

Because Rückgabetermin is a masculine noun.

A very useful rule for German compounds is: the last part usually decides the gender.
Here, the last part is Termin, and you say:

• der Termin

So the full compound is also masculine:

• der Rückgabetermin

In this sentence, it is also the subject, so the nominative article der is the correct form.

Because German very often combines nouns into a single compound noun.

This word is made from:

• Rückgabe = return
• Termin = date / appointment / scheduled time

So German makes one word:

• Rückgabetermin

English usually writes these ideas as separate words, such as return date or due date, but German prefers one compound noun.

It is in the nominative case.

That is because der Rückgabetermin is the subject of the sentence — it is the thing being talked about.

You can see the structure like this:

• Der Rückgabetermin = subject
• ist = verb
• morgen = time adverb

So the noun phrase stays in the nominative:

• der Rückgabetermin

Because German main clauses usually follow the verb-second rule.

That means:

• one element comes first
• the finite verb comes second

In this sentence:

• Der Rückgabetermin | ist | morgen

If you move another element to the front, the verb still stays second:

• Morgen ist der Rückgabetermin.

So the verb position is very important in German sentence structure.

Because morgen here is an adverb meaning tomorrow.

German often uses time adverbs with no preposition:

• heute = today
• morgen = tomorrow
• gestern = yesterday

So:

• Der Rückgabetermin ist morgen.

is completely normal.

If you were talking about the morning, that would be different:

• am Morgen = in the morning

So morgen and Morgen are not the same thing here.

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

In German:

• nouns are capitalized
• adverbs are not

Compare:

• morgen = tomorrow
• der Morgen = the morning

If the word comes at the beginning of the sentence, it will be capitalized only because it starts the sentence:

• Morgen ist der Rückgabetermin.

Yes, absolutely.

Both of these are correct:

• Der Rückgabetermin ist morgen.
• Morgen ist der Rückgabetermin.

The difference is mainly focus:

• Der Rückgabetermin ist morgen. → starts with the subject
• Morgen ist der Rückgabetermin. → puts more emphasis on morgen

This is very common in German: you can move elements around, but the conjugated verb still stays in second position.

It tells you what kind of Termin it is.

German compounds often work this way:

• the last word gives the basic category
• the earlier part gives more information

So here:

• Termin = the main idea
• Rückgabe = specifies it

That means Rückgabetermin is literally a return-related date/appointment/deadline.

This pattern is extremely common in German vocabulary.

Termin is a flexible word in German. Depending on context, it can mean:

• appointment
• scheduled date
• time slot
• deadline

In Rückgabetermin, it refers to the scheduled date for returning something, so English often uses:

• return date
• due date

So while Termin often means appointment, in compounds it can have a slightly broader meaning.

A rough English-friendly guide is:

• RUEK-gah-buh-tehr-MEEN

A few pronunciation notes:

• Rück- contains ü, which does not exist in normal English pronunciation.
  Try saying ee with your tongue, but round your lips as if saying oo.
• gabe sounds roughly like gah-buh
• Termin is stressed on the last syllable: tehr-MEEN

So the whole word is said in connected parts:

• Rück-gabe-Termin

If you pronounce the three parts clearly, German speakers will usually understand you.