Learn Hardcore Swedish: Den blå kostymen och den röda slipsen passar bra ihop. - The blue suit and the red tie go well together.

The -en at the end of kostymen and slipsen is the definite singular ending for common-gender nouns.
When there is an adjective in front of a definite noun, Swedish also adds a separate definite article (den, det, or de) before the adjective.

So:

kostym → kostymen (the suit)
blå kostym → den blå kostymen (the blue suit)

The same pattern applies to den röda slipsen. This is normal and required in standard Swedish.

Why is it den and not det or de? How do I know which one to use?

Den, det, and de are the definite articles that agree with the noun’s gender and number:

den – for common gender (en-words) in singular
det – for neuter gender (ett-words) in singular
de – for plural, regardless of gender

Here, kostym and slips are en-words, so in the definite form with an adjective they take den:

den blå kostymen
den röda slipsen

If it were a neuter noun, you’d say, for example, det blå huset (the blue house).

Why is the verb passar used with two things (kostymen och slipsen) but doesn’t change form? Shouldn’t it be “passar” vs “passar?”

In Swedish, present tense verbs do not change form for singular vs. plural. The same form is used for I, you, he/she/it, and they.

So:

Jag passar
Han passar
De passar

All use passar.
In this sentence, den blå kostymen och den röda slipsen form a compound subject, but the verb still simply appears as passar. There is no separate plural ending.

What exactly does passar bra ihop mean? Is ihop necessary, and how is this different from just passar bra?

passa bra by itself means something like “suit / fit / be suitable (well)”.
passa bra ihop adds the sense of “fit well together / go well together / match”.

In this sentence, we are talking about how the two items work together as a combination, not whether each one individually fits someone.

Compare:

Kostymen passar bra. – The suit fits / suits (someone) well.
Kostymen och slipsen passar bra ihop. – The suit and the tie go well together / match well.

So ihop is what gives you the idea of togetherness / combination.

What does ihop literally mean here, and can I replace it with tillsammans?

ihop literally has the sense of “together / in one piece / as a unit”. In combinations like passa ihop, it forms a particle verb meaning “to match / to go together”.

You can say:

Den blå kostymen och den röda slipsen passar bra tillsammans.

This is understandable and fine, but passa (bra) ihop is the most idiomatic collocation when you talk about colors, clothes, furniture, etc. matching. tillsammans is more neutral “together” and isn’t as fixed with passa as ihop is.

Why is it blå in den blå kostymen but röda in den röda slipsen? Why don’t they both end in -a or both stay unchanged?

Both adjectives are in their definite form, but they behave slightly differently:

röd → röda in the definite form with den:
- en röd slips
- den röda slipsen
blå is a bit special: with den
- common gender in the singular, it often stays as blå:
- en blå kostym
- den blå kostymen

So you see two definite adjectives, but one of them (röd) shows it more clearly by adding -a, while blå looks the same as in the basic form in this context. (It still changes in other forms, e.g. ett blått hus, blåa hus.)

Could I say den blåa kostymen instead of den blå kostymen?

Yes, den blåa kostymen is also heard in modern Swedish, and many speakers accept both den blå kostymen and den blåa kostymen.

However, in more formal or traditional written Swedish, den blå kostymen (without -a) is often preferred for blå. As a learner, you’re perfectly safe using den blå kostymen; it sounds natural and fully correct.

Why do we repeat den before both nouns (den blå kostymen och den röda slipsen)? Could we say den blå kostymen och röda slipsen instead?

In Swedish, when you coordinate two separate definite noun phrases with adjectives, you normally repeat the article:

den blå kostymen och den röda slipsen

This makes it clear that each item is definite in its own right.

You could technically say den blå kostymen och röda slipsen, but that tends to sound elliptical or slightly off in standard written Swedish, as if you had left out something. The natural and recommended form is to keep den in front of each adjective–noun unit.

How would the meaning change if I said Blå kostym och röd slips passar bra ihop without den and without -en endings?

Without the definite markers, the phrase becomes indefinite and more generic:

Blå kostym och röd slips passar bra ihop.
→ A blue suit and a red tie (in general) go well together.

This sounds like a general statement about color combinations or style.

With the definites:

Den blå kostymen och den röda slipsen passar bra ihop.
→ The blue suit and the red tie (we have in mind / can see) go well together.

Now you’re clearly referring to specific items.

Could I just say Kostymen och slipsen passar bra ihop and leave out the colors?

Yes, that’s grammatically correct:

Kostymen och slipsen passar bra ihop.

This would mean “The suit and the tie go well together”, but without saying which suit or tie, or which colors they are.

Adding the adjectives (den blå kostymen, den röda slipsen) gives more specific and descriptive information and makes it easier to identify the exact items.

Where would I put inte (not) if I want to say they do not go well together?

In a main clause like this, inte normally comes after the finite verb (passar) and before adverbs like bra:

Den blå kostymen och den röda slipsen passar inte bra ihop.

The order is:

Subject: Den blå kostymen och den röda slipsen
Verb: passar
Negation: inte
Adverb: bra
Particle: ihop

Is the word order Den blå kostymen och den röda slipsen passar bra ihop typical for Swedish main clauses? Why is the verb not in second place here?

It actually is typical: Swedish has V2 word order in main clauses, which means the finite verb is in the second position of the clause, not necessarily the second word.

Here, the entire subject phrase:

Den blå kostymen och den röda slipsen

counts as the first position, and passar is in second position after that phrase. So the structure fully obeys the normal Swedish main-clause word order:

Subject: Den blå kostymen och den röda slipsen
Verb: passar
Rest of the clause: bra ihop