Í mátunarklefanum sá hún að hettupeysan passaði betur en regnfrakkinn.

Because í takes different cases depending on meaning:

• location: usually dative
• motion into something: usually accusative

Here the meaning is in the fitting room as a location, so Icelandic uses the dative: í mátunarklefanum.

Also, -num includes the definite article:

• mátunarklefi = fitting room
• mátunarklefanum = in the fitting room

So this form is both dative singular and definite.

This is because Icelandic follows the V2 rule in main clauses: the finite verb usually comes in the second position.

The sentence begins with Í mátunarklefanum. Once that phrase takes the first position, the verb must come next:

• Í mátunarklefanum sá hún ...

If you started with the subject instead, then you would get:

• Hún sá ...

Both are possible, but the sentence you have emphasizes the setting first: in the fitting room.

No. Here sá is the past tense of sjá (to see).

So in this sentence:

• sá hún = she saw

Icelandic has another word also spelled sá, which can be a demonstrative/pronoun. The meaning depends on grammar and context. Here it is clearly a verb.

Að introduces a subordinate clause, like English that:

• sá hún að ... = she saw that ...

So the structure is:

• main clause: Í mátunarklefanum sá hún
• subordinate clause: að hettupeysan passaði betur en regnfrakkinn

In English, that is often omitted, but in Icelandic að is very commonly used in this kind of sentence.

Because hettupeysan is the definite form: the hoodie.

Breakdown:

• hettupeysa = hoodie
• hettupeysan = the hoodie

It is also in the nominative singular, because it is the subject of the subordinate clause:

• hettupeysan passaði ... = the hoodie fit ...

This word is a compound:

• hetta = hood
• peysa = sweater / sweatshirt

So hettupeysa literally means something like hood-sweater, i.e. hoodie.

Here passaði means fit or suited.

It is the past tense of passa. In clothing contexts, passa often means:

• to fit
• to suit
• to go well

So hettupeysan passaði betur means the hoodie fit better or suited her better.

This is a very useful verb, because passa can have several meanings depending on context.

Because betur is the adverb form, while betri is the adjective form.

Here betur modifies the verb passaði:

• passaði betur = fit better

Compare:

• betri peysa = a better sweater → adjective
• passaði betur = fit better → adverb

So Icelandic uses betur because the sentence is describing how it fit.

Because regnfrakkinn is understood as the subject of an omitted verb:

• hettupeysan passaði betur en regnfrakkinn (passaði)

In other words, it means:

• the hoodie fit better than the raincoat did

That is why regnfrakkinn stays in the nominative, not the accusative.

If you are an English speaker, it may help to think of it as an abbreviated comparison where Icelandic leaves out the repeated verb.

Because the sentence is talking about a specific raincoat, not just any raincoat.

• regnfrakki = a raincoat
• regnfrakkinn = the raincoat

The same is true for hettupeysan: both items are specific garments being compared.

Icelandic usually attaches the definite article to the end of the noun instead of using a separate word like English the.

Examples from this sentence:

• mátunarklefanum = the fitting room (in the fitting room)
• hettupeysan = the hoodie
• regnfrakkinn = the raincoat

So one big adjustment for English speakers is learning to recognize these noun endings as including the.