Þessi trefill passar vel við húfuna.

Because trefill is a masculine singular noun, and the demonstrative þessi has to agree with it in gender, number, and case.

Here:

• þessi = this for a masculine/feminine noun in the nominative singular
• trefill = a masculine singular noun
• the whole phrase Þessi trefill means this scarf

Compare:

• þessi trefill = this scarf
• þetta is used with neuter nouns, not with trefill

So the form þessi is chosen because it matches trefill.

Trefill is in the nominative singular.

It is the subject of the sentence — the thing doing the action of matching / going well with something.

So:

• Þessi trefill = subject
• passar = verb
• við húfuna = prepositional phrase, with the hat

In Icelandic, subjects are very often in the nominative case, just like here.

Here passar við means goes well with, matches, or suits.

So the idea is not really about physical size or fitting into something. It is about appearance or compatibility:

• Þessi trefill passar við húfuna = This scarf matches the hat
• with vel, it becomes This scarf matches the hat well

A useful thing to know is that passa can have several meanings in Icelandic depending on context, for example:

• fit
• suit
• match
• sometimes even be careful / watch out, in other expressions

But in this sentence, passa við clearly means match / go well with.

Vel means well.

It is an adverb, and it modifies the verb phrase:

• passar við = matches / goes with
• passar vel við = matches well / goes well with

Its position here is very natural:

• Þessi trefill passar vel við húfuna.

So literally, the structure is roughly:

• This scarf matches well with the hat

In smoother English, you would usually say:

• This scarf goes well with the hat
• This scarf matches the hat well

Because húfuna is the definite accusative singular form of húfa.

Breakdown:

• húfa = a hat / hat
• húfuna = the hat

In Icelandic, the definite article is usually attached to the end of the noun instead of being a separate word like English the.

So:

• húfa = a hat
• húfan = the hat, nominative
• húfuna = the hat, accusative

In this sentence, the form needs to be húfuna, not húfan, because of the preposition við, which takes the accusative here.

In this expression, við is followed by the accusative.

That is why we get:

• við húfuna = with the hat

The noun húfa changes form because it is the object of the preposition:

• nominative: húfan
• accusative: húfuna

For learners, the most practical approach is often to learn the whole pattern:

• passa við + accusative = to match / go with

So instead of memorizing only passa, it is useful to remember:

• eitthvað passar við eitthvað
• something goes with something

Not exactly, even though with is often a good translation.

In this sentence, við expresses the idea of:

• with
• against
• in relation to
• or more idiomatically here, goes with / matches

So while við húfuna can be translated as with the hat, the whole phrase passar vel við húfuna is better understood as:

• goes well with the hat
• matches the hat well

This is a good example of why it is often better to learn the whole expression instead of translating each word one by one.

Þessi begins with the Icelandic letter þ, which is pronounced like the th in thin, not like the th in this.

A rough guide:

• Þessi ≈ THESS-i

More specifically:

• þ = voiceless th
• e = like e in bet
• double ss = a clear s sound
• final i = a short i sound

So an English speaker should avoid pronouncing þ like z, d, or the voiced th in this.

A rough English-friendly pronunciation is:

THESS-i TREF-fill PASS-ar vel vith HOO-vu-na

A few notes:

• Þ in Þessi = th as in thin
• tr in trefill is pronounced clearly
• ll in trefill is just a normal l sound here
• a in passar is short
• við ends with ð, which is like the th in this
• ú in húfuna is a long oo sound

This is only an approximation, but it will point you in the right direction.

Because this sentence uses a very straightforward subject–verb–adverb–prepositional phrase order:

• Þessi trefill = subject
• passar = verb
• vel = adverb
• við húfuna = prepositional phrase

So the structure is close to English:

• This scarf matches well with the hat

Icelandic word order can be more flexible than English, but simple main clauses often look quite familiar to English speakers.

Yes.

You can say:

• Þessi trefill passar við húfuna.

That means:

• This scarf matches the hat
• This scarf goes with the hat

Adding vel makes it stronger or more natural in many contexts:

• Þessi trefill passar vel við húfuna.
• This scarf goes well with the hat

So vel is not required for grammar, but it adds the idea of well.

The dictionary forms are:

• trefill = scarf
• húfa = hat, cap, beanie

In dictionaries, Icelandic nouns are usually listed in the nominative singular indefinite form.

So:

• trefill is already in its dictionary form
• húfuna is not; its dictionary form is húfa

This is important because learners often need to recognize inflected forms and trace them back to the base noun.

Because the ending includes the suffixed definite article.

In Icelandic, the is usually attached to the noun:

• húfa = a hat / hat
• húfan = the hat
• húfuna = the hat, in the accusative

So the -na at the end is part of the definite form here.

That is why við húfuna means with the hat, not with a hat.