Beltið mitt passar ekki við þennan kjól.

Here passa við means to match, to go with, or to suit in the sense of appearance, color, or style.

So Beltið mitt passar ekki við þennan kjól means that the belt and the dress do not look right together.

A useful contrast:

• Beltið passar. = The belt fits.
• Beltið passar við kjólinn. = The belt matches the dress.

The preposition við changes the meaning.

Because Icelandic normally uses passa við for the idea of matching or going with something.

Even though English uses with in go with this dress, Icelandic does not usually use með in this expression.
So:

• passa við = match / go with
• með usually means with in other senses, such as accompaniment, instrument, or possession

So this is something you should learn as a set phrase: passa við + accusative.

Because þennan agrees with kjól, and kjól is:

• masculine
• singular
• accusative

The preposition við takes the accusative here, so this dress becomes þennan kjól.

The dictionary form is þessi = this, but it changes form depending on gender, number, and case.

For masculine singular:

• nominative: þessi
• accusative: þennan

That is why you get þennan kjól.

Because kjóll is the nominative singular form, but here the noun is in the accusative singular after við.

So the forms are:

• nominative: kjóll
• accusative: kjól

This is very common in Icelandic: the dictionary form of a noun is often not the form you need in an actual sentence.

Because Icelandic often puts possessive pronouns after the noun.

So:

• beltið mitt = my belt

This pattern is extremely common. In this structure, the noun usually has the suffixed definite article:

• húsið mitt = my house
• bíllinn minn = my car
• beltið mitt = my belt

You can also say mitt belti, but that often sounds more emphatic, contrastive, or stylistically marked. For many everyday sentences, beltið mitt is the more natural choice.

Because Icelandic commonly uses the suffixed definite article with a noun when the possessive comes after it.

So literally:

• beltið = the belt
• mitt = my

Together: beltið mitt = my belt

This may feel strange to an English speaker, but it is normal Icelandic grammar. English and Icelandic simply structure possession differently here.

Because mitt has to agree with belti.

Belti is a neuter noun, so the possessive pronoun must also be neuter.

Compare:

• masculine: minn
• feminine: mín
• neuter: mitt

So:

• minn bíll / bíllinn minn = my car
• mín bók / bókin mín = my book
• mitt belti / beltið mitt = my belt

Here the noun is neuter singular, so mitt is correct.

Because in a normal main clause, Icelandic usually puts the finite verb early in the sentence, and ekki typically comes right after that verb.

So:

• Beltið mitt passar ekki ...

This is the normal order.

A very useful rule of thumb is:

• subject
• finite verb
• ekki
• the rest

This is one of the places where Icelandic word order differs from English.

Because grammatical gender in Icelandic is not the same thing as real-world sex or gender.

The noun kjóll is grammatically masculine, even though it refers to an item of clothing often associated with women.

That is why you get masculine agreement:

• þennan kjól

This is normal. Many Icelandic nouns have grammatical gender that does not match what an English speaker might expect.

Because when Icelandic uses a demonstrative like þessi / þennan / þetta, the noun usually does not also take the suffixed definite article.

So:

• þennan kjól = this dress

not normally:

• þennan kjólinn

The demonstrative already makes the noun definite, so another definite marker is usually unnecessary.

Not really in normal usage.

Because the sentence uses passa við, the meaning is about matching in appearance or style, not physical fit.

So the natural interpretation is:

• the belt does not go well with the dress
• the belt does not match the dress

If you wanted to say something about physical fit, you would normally use passa without við, or use a different sentence altogether depending on the situation.