Min nye lejlighed er større end den gamle.

Because lejlighed is a common-gender noun: en lejlighed. With singular common-gender nouns, the possessive is min.

• min lejlighed
• mit hus
• mine bøger

So min nye lejlighed is correct.

After a possessive like min, Danish normally uses the -e form of the adjective. So ny becomes nye in min nye lejlighed.

Compare:

• en ny lejlighed = an new apartment
• min nye lejlighed = my new apartment
• lejligheden er ny = the apartment is new

So the form changes depending on how the adjective is used.

Because the possessive min already makes the noun phrase definite in meaning. Danish does not normally combine a possessive with the definite ending.

So you say:

• min nye lejlighed

Not:

• min nye lejligheden
• den min nye lejlighed

This is a very common pattern in Danish.

Større is the comparative form of stor, meaning bigger / larger.

The pattern is:

• stor = big
• større = bigger
• størst = biggest

This is not fully regular, so it is best learned as a set. Danish does not normally say mere stor for bigger.

Because end is the word used after a comparative, just like than in English.

• større end = bigger than
• mindre end = smaller than

Do not confuse it with som, which is used in equal comparisons:

• lige så stor som = as big as

So:

• større end den gamle = bigger than the old one

Because lejlighed is a common-gender noun, and common-gender nouns take den.

• en lejlighed → den gamle
• et hus → det gamle

So den gamle means the old one referring back to lejlighed.

Because it is already understood from the context. Danish often leaves out a repeated noun when it is obvious.

So these are both possible:

• Min nye lejlighed er større end den gamle.
• Min nye lejlighed er større end den gamle lejlighed.

The shorter version is very natural.

It can suggest either, depending on context. In a sentence about apartments, den gamle often means the old one in the sense of the previous one, not necessarily a very aged apartment.

So the exact nuance comes from the situation, not just the grammar.

Because gamle is simply identifying which apartment you mean: the old one.

Ældre is the comparative form of gammel and means older. That would be used if you were comparing age.

Here, the sentence compares size, not age:

• større = the comparison
• den gamle = the apartment being compared with

So gamle is the natural choice.

No. The comparative form større stays the same.

For example:

• en større lejlighed
• et større hus
• større lejligheder
• lejligheden er større

So once you have the comparative form større, you usually keep that same form.

Because this is a normal Danish main clause. The subject comes first, and the finite verb er comes in second position.

So the basic structure is:

• Min nye lejlighed = subject
• er = finite verb
• større end den gamle = rest of the sentence

This is the standard word order for a simple statement in Danish.