Den här jackan är för tunn på vintern.

Den här means this for a common-gender noun.

• den = the common-gender demonstrative part
• här = here, which helps create the meaning this
• jackan = the jacket

So den här jackan literally looks a bit like this the jacket, but in natural Swedish it simply means this jacket.

For comparison:

• den här jackan = this jacket
• det här huset = this house

You use den with common-gender nouns and det with neuter nouns.

When you say this jacket in Swedish, the noun normally takes the definite form:

• jacka = a jacket
• jackan = the jacket

So Swedish usually says:

• den här jackan = this jacket

not:

• den här jacka ❌

This is a very common pattern in Swedish: after den här / det här / de här, the noun is usually definite.

Because tunn here comes after the verb är, so it is a predicate adjective. Predicate adjectives agree with the noun in gender and number.

Since jackan is a singular common-gender noun, the basic form tunn is correct:

• jackan är tunn = the jacket is thin

Compare:

• huset är tunt = the house is thin/light
• jackorna är tunna = the jackets are thin

So the adjective does change in other contexts, but for jackan in singular common gender, tunn is the right form.

För means too in the sense of excessively.

So:

• för tunn = too thin

This is different from English for. In this sentence, för is not a preposition like for winter. It modifies the adjective and means that the jacket is more thin than is suitable.

Other examples:

• för kallt = too cold
• för dyr = too expensive
• för liten = too small

Swedish often uses på with seasons when talking about something happening during that season in a general sense.

So:

• på vintern = in winter / during the winter

This is an idiomatic Swedish expression. English learners often expect i, but på vintern is the normal choice here.

Similar expressions:

• på sommaren = in the summer
• på hösten = in the autumn
• på våren = in the spring

In expressions like på vintern, Swedish usually uses the definite form of the season:

• vintern = the winter

But in English, we usually translate it simply as in winter or during winter, not necessarily in the winter.

This is just how Swedish commonly expresses general time periods in these seasonal phrases:

• på vintern
• på sommaren
• på hösten
• på våren

So even though the noun is definite in Swedish, the English translation may sound more natural without the.

If you want to say this jacket, then yes, här is part of the normal expression.

• den här jackan = this jacket

Without här, den jackan usually means that jacket or that particular jacket, depending on context.

So:

• den här jackan = this jacket
• den jackan = that jacket / that jacket there / that particular jacket

For learners, it is best to remember den här as a fixed pattern for this.

Yes, denna jacka is possible, but it sounds more formal or written than den här jackan.

Compare:

• den här jackan = common, everyday spoken Swedish
• denna jacka = more formal, written, or stylistic

A native speaker in ordinary conversation would very often prefer den här jackan.

The dictionary form is tunn.

Its common forms are:

• tunn = common gender singular
• tunt = neuter singular
• tunna = plural and definite form in many adjective uses

Examples:

• en tunn jacka = a thin jacket
• ett tunt tyg = a thin fabric
• två tunna jackor = two thin jackets

In your sentence, jackan is singular common gender, so tunn is correct.

This is standard Swedish main-clause word order:

• Den här jackan = subject
• är = verb
• för tunn = complement
• på vintern = time expression

So the structure is:

subject + verb + complement + time phrase

Very literally:

• Den här jackan = this jacket
• är = is
• för tunn = too thin
• på vintern = in winter

This is straightforward main-clause order and is very similar to English here.

Then both the demonstrative and the adjective would change.

For example, with ett täcke (a blanket, neuter):

• Det här täcket är för tunt på vintern.

Changes:

• den här → det här
• jackan → täcket
• tunn → tunt

So neuter nouns affect both the this expression and the adjective form after är.