Hon säger att den blå skjortan fortfarande är hennes favorit, fastän den är gammal.

Because Swedish usually uses double definiteness when a definite noun has an adjective in front of it.

• skjorta = shirt
• skjortan = the shirt
• den blå skjortan = the blue shirt

You need:

• the definite article before the adjective: den
• the noun in its definite form: skjortan

So den blå skjortan is the normal way to say the blue shirt.

Because skjorta is a common gender noun in Swedish, not a neuter one.

• common gender nouns use en in the indefinite form and den in the definite/article form
• neuter nouns use ett and det

So:

• en skjorta
• den skjortan
• den blå skjortan

If the noun were neuter, you would use det instead.

The adjective blå stays as blå here because it is modifying a common gender singular definite noun.

In Swedish, adjectives change depending on gender, number, and definiteness. For blå, some common forms are:

• en blå skjorta = a blue shirt
• ett blått hus = a blue house
• blå skjortor = blue shirts
• de blå skjortorna = the blue shirts

In den blå skjortan, blå is the correct form for a definite singular common-gender noun.

Because this part of the sentence is a subordinate clause introduced by att.

In Swedish subordinate clauses, sentence adverbs and similar words often come before the finite verb.

So:

• Hon säger att den blå skjortan fortfarande är hennes favorit.

Compare that with a main clause, where the finite verb usually comes earlier:

• Den blå skjortan är fortfarande hennes favorit.

This is a very common word-order difference in Swedish.

fastän means although, even though, or though.

Here it introduces a contrast:

• Hon säger att den blå skjortan fortfarande är hennes favorit, fastän den är gammal.
• She says the blue shirt is still her favorite, even though it is old.

It works much like English although. You may also see trots att or även om in similar contexts, but fastän is a very natural choice here.

That den is a pronoun meaning it. It refers back to den blå skjortan.

Since skjorta is a common-gender noun, the pronoun is den:

• Skjortan är gammal. Den är gammal.

If the noun were neuter, Swedish would use det instead.

So this den is not the article the. It is the pronoun it.

Because hennes refers to her in the sense of the woman being talked about: Hon säger ... hennes favorit.

Swedish sin/sitt/sina is a reflexive possessive, used when the possessor is the subject of the same clause.

Here, the relevant clause is:

• den blå skjortan fortfarande är hennes favorit

The subject of that clause is den blå skjortan, not hon. So you do not use sin here.

Compare:

• Hon älskar sin skjorta. = She loves her own shirt.
  (subject hon owns the shirt in the same clause)
• Hon säger att skjortan är hennes favorit. = She says that the shirt is her favorite.
  (use hennes, not sin)

Not in this sentence as written.

Because the clause begins with att, it is subordinate, and the normal word order is:

• subject + adverb + finite verb

So:

• att den blå skjortan fortfarande är hennes favorit

In a main clause, you could say:

• Den blå skjortan är fortfarande hennes favorit.

So both patterns exist, but they belong to different clause types.

favorit means favorite. In this sentence, hennes favorit means her favorite one or her favorite item.

Swedish often leaves the noun implied when it is obvious from context. So:

• den blå skjortan är hennes favorit

naturally means:

• the blue shirt is her favorite
  or
• the blue shirt is her favorite one

You do not need to repeat skjorta.

Yes, in this sentence fortfarande means still.

• fortfarande är hennes favorit = is still her favorite

It shows that this has continued to be true over time.

A learner may also see ännu, which can sometimes overlap with fortfarande, but fortfarande is very common for ongoing situations like this one.

The comma separates the main statement from the contrasting clause introduced by fastän.

Swedish punctuation is sometimes a bit more flexible than English punctuation, but a comma here is very natural and helps readability:

• Hon säger att den blå skjortan fortfarande är hennes favorit, fastän den är gammal.

It marks the extra idea:

• it is her favorite
• despite the fact that it is old

Yes, gammal is completely normal for objects too.

It can mean:

• old in age
• not new
• worn from age

So den är gammal here simply means it is old. For a shirt, that could mean it has been owned for a long time or is no longer new.

Not in standard Swedish in this structure.

After säger when introducing a full clause, Swedish normally uses att:

• Hon säger att den blå skjortan fortfarande är hennes favorit.

This is like English that in She says that.... English can often omit that, but Swedish usually keeps att in sentences like this.