Learn Hardcore Swedish: Den blå kostymen med röd slips passar honom mycket bättre. - The blue suit with a red tie suits him much better.

*det blå kostymen – wrong gender
*den blå kostym – mixing a definite article (den) with an indefinite noun form (kostym). Correct would be en blå kostym (a blue suit) or den blå kostymen (the blue suit).

Why do we have both den and -en in den blå kostymen? Isn’t that double definiteness?

Yes, this is the so‑called double definiteness, and it is normal in standard Swedish when you have:

definite article + adjective + definite noun

Pattern:

den röda bilen – the red car
det stora huset – the big house
de nya böckerna – the new books

In your sentence:

den – definite article for an en-word
blå – adjective
kostymen – noun with definite ending -en

So den blå kostymen follows the regular rule. You must keep both den and -en here; you cannot drop one of them.

Why is it med röd slips and not med en röd slips?

Both are grammatically possible:

med röd slips
med en röd slips

In descriptions of clothing, body parts, etc., Swedish very often uses a bare indefinite noun (without en/ett) after med:

en man med skägg – a man with a beard
en kvinna med glasögon – a woman with glasses
en kille med röd slips – a guy with a red tie

So med röd slips feels natural and idiomatic: with (a) red tie.

If you say med en röd slips, it is still correct, but it can sound a bit more “countable” or specific, slightly emphasizing a single red tie as an item.
You could also have:

med den röda slipsen – with the red tie (a particular one that you and the listener know about)

Why is it röd and not röda in med röd slips?

Adjectives in Swedish agree with the noun in definiteness, number and gender. For röd:

Singular, common gender, indefinite: röd
- en röd slips – a red tie
Singular, neuter, indefinite: rött
- ett rött hus – a red house
All definite forms and all plurals: röda
- den röda slipsen – the red tie
- röda slipsar – red ties
- de röda slipsarna – the red ties

In med röd slips, slips is singular, common gender, indefinite, so the adjective is in its base form: röd.

What exactly does passar mean here, and how is passa used with clothes?

passa has several related meanings in Swedish; with clothes it typically means to fit / to suit (someone).

In your sentence:

Den blå kostymen med röd slips passar honom mycket bättre.
→ The blue suit with a red tie suits him much better (looks better on him / is more suitable for him).

Common uses with clothes:

Kostymen passar honom. – The suit fits him.
Kjolen passar henne inte. – The skirt doesn’t fit her.
De här skorna passar mig. – These shoes fit me.

More generally, passa can also mean:

to be suitable / convenient: Den tiden passar mig bra. – That time suits me well.
to look after: Kan du passa barnen? – Can you look after the children?

Here it’s the clothing meaning: fit / suit (someone).

Why do we say passar honom and not passar han?

Swedish, like English, distinguishes subject and object forms of personal pronouns:

han = he (subject)
honom = him (object)

The verb passa någon means to suit someone; någon is the object of the verb. So you need the object form honom:

Det passar honom. – It suits him.
Kläderna passar henne. – The clothes suit her.

If he is the subject, you use han:

Han passar i den blå kostymen. – He looks good in the blue suit.

So:

Den blå kostymen … passar honom … – The blue suit … suits him …
not
*passar han – wrong case.

Is the word order passar honom mycket bättre fixed, or can I move the words around?

The natural word order here is:

[Subject] Den blå kostymen med röd slips
[Verb] passar
[Object] honom
[Adverbial] mycket bättre

→ Den blå kostymen med röd slips passar honom mycket bättre.

Some important points:

Swedish main clauses normally have the finite verb in second position (V2).
Here, the whole subject phrase Den blå kostymen med röd slips is position 1, and passar is position 2.
After the verb, the object (honom) typically comes before adverbials like mycket bättre in a sentence like this.

You would not normally say:

*Den blå kostymen med röd slips honom passar mycket bättre. – wrong / very odd
*Den blå kostymen med röd slips passar mycket bättre honom. – sounds wrong or at least very unnatural in standard Swedish.

You can change other parts, but then the whole structure changes, for example:

För honom passar den blå kostymen med röd slips mycket bättre.
(Here För honom is in first position, then passar is still second.)

How does mycket bättre work, and why not mer bra?

bra (good) has an irregular comparative:

bra – good
bättre – better
bäst – best

You do not say *mer bra for “more good”. You must use bättre.

mycket is an adverb here meaning much / a lot, and it intensifies the comparative:

bättre – better
mycket bättre – much better / a lot better
lite bättre – a bit better

So:

mycket bättre = much better
not *mer bra (ungrammatical in this meaning)

Why do we use mycket and not många before bättre?

Swedish distinguishes between:

mycket – much / a lot (for uncountable things and adjectives/adverbs)
många – many (for countable nouns)

Examples:

mycket vatten – much water
många bilar – many cars
mycket stor – very big / much bigger (with a comparative: mycket större)

Since bättre is an adjective (comparative), you use mycket to intensify it:

mycket bättre – much better

många bättre would be wrong here, because bättre is not a countable noun.

How would the sentence change if I talk about more than one suit?

If you mean several blue suits with red ties, you make both the subject and (probably) ties plural:

De blå kostymerna med röda slipsar passar honom mycket bättre.

Changes:

Den → De
- den = the (singular, common gender)
- de = the (plural)
kostymen → kostymerna
- kostym – suit
- kostymer – suits
- kostymerna – the suits
röd slips → röda slipsar
- en röd slips – a red tie
- röda slipsar – red ties (indefinite plural)

The rest stays the same:

passar honom mycket bättre – suits him much better.

What is the difference between med röd slips and i röd slips?

Both med and i can appear in descriptions of clothing, but they’re used a bit differently.

med (with): very common with accessories and also clothes in general
- en man med röd slips – a man with a red tie
- en kvinna med hatt – a woman with a hat
i (in): especially common for clothes you are “inside” of – dresses, coats, jackets, etc., focusing on being dressed in something
- en man i blå kostym – a man in a blue suit
- en tjej i röd klänning – a girl in a red dress

With slips, med röd slips is the natural expression.
i röd slips would sound unusual; people normally say med slips, med röd slips.

Do adjectives always go before the noun like in den blå kostymen, or can they come after?

They can do both, but the form changes a bit.

Before the noun (attributive use) – as in your sentence
- en blå kostym – a blue suit
- den blå kostymen – the blue suit
After a verb (predicative use)
Here the adjective comes after är, blir, verkar, etc.:
- Kostymen är blå. – The suit is blue.
- Slipsen är röd. – The tie is red.
- Den här kostymen verkar bättre. – This suit seems better.

In the predicative position, you don’t use den / det / de in front of the adjective like in den blå kostymen. The definiteness is carried only by the noun (kostymen), not repeated before the adjective.

Does the whole sentence change if I remove med röd slips?

No structural change is needed; you simply drop that prepositional phrase:

Full sentence:
Den blå kostymen med röd slips passar honom mycket bättre.
Without med röd slips:
Den blå kostymen passar honom mycket bättre.

The grammar and word order remain the same:

Den blå kostymen – subject
passar – verb
honom – object
mycket bättre – adverbial

You just lose the extra detail about the red tie.