Hún skoðaði tvær íbúðir í gær, en hvorug þeirra var jafn góð og sú sem kostar níutíu þúsund.

Skoðaði is the past tense of skoða (to look at / inspect / view). The sentence is talking about something she did yesterday, so the past tense is required.

• skoða = to look at, inspect
• skoðaði = looked at / inspected

So Hún skoðaði... í gær means she viewed or inspected them yesterday.

Because Icelandic numbers agree with the noun in gender and often also in case.

Íbúð is a feminine noun, and here it is the direct object of skoðaði, so it is in the accusative plural. The feminine form of two in this context is tvær.

Compare:

• tveir = two (masculine)
• tvær = two (feminine)
• tvö = two (neuter)

So:

• tvær íbúðir = two apartments

Íbúðir is the plural of íbúð (apartment), and here it is the object of the verb skoðaði.

For this noun, the nominative plural and accusative plural happen to look the same:

• singular: íbúð
• plural: íbúðir

So even though it is a direct object, the form you see is still íbúðir.

Í gær means yesterday.

It is a fixed expression in Icelandic. Learners often notice that it literally contains í, which usually means in, but in this expression you should treat í gær as a whole time phrase.

So:

• í dag = today
• í gær = yesterday
• á morgun = tomorrow

Hvorug þeirra means neither of them.

It is made of:

• hvorug = neither one (feminine singular here)
• þeirra = of them

So the structure is very similar to English neither of them.

The reason hvorug is feminine singular is that it refers back to an understood singular noun: íbúð. In other words, the idea is neither apartment.

Because Icelandic often uses the genitive after words like neither, both, many, and similar expressions.

Here:

• þau / þær / þeir = they
• þeirra = of them

So hvorug þeirra literally works like neither of them.

This is a very common pattern, and it is worth learning as a chunk.

Because the subject of that clause is not two apartments. The subject is hvorug þeirra = neither of them.

Grammatically, hvorug is singular, so the verb is singular too:

• hvorug ... var = neither ... was

Even though two apartments are being discussed overall, the grammar treats neither one as a singular subject.

Góð is the feminine singular form of góður (good).

It matches the subject hvorug, which stands for íbúð (apartment), a feminine noun.

So the agreement is:

• íbúð = feminine singular
• hvorug = feminine singular
• góð = feminine singular

This is why the sentence has var jafn góð.

Jafn ... og is the Icelandic pattern for as ... as.

So:

• jafn góð og = as good as

Examples:

• Hann er jafn hár og ég. = He is as tall as I am.
• Hún er jafn góð og systir sín. = She is as good as her sister.

In your sentence, it means neither of the two apartments was as good as the other one being mentioned.

Sú sem means the one that or the one which.

• sú = that one / the one (feminine singular)
• sem = that / which / who

Because íbúð is feminine, the sentence uses sú.

So:

• sú sem kostar níutíu þúsund = the one that costs ninety thousand

This is a very common Icelandic way to avoid repeating the noun.

Because sú is a demonstrative pronoun meaning that one / the one, and it agrees with the hidden noun íbúð, which is feminine singular.

Compare:

• sá = that one (masculine)
• sú = that one (feminine)
• það = that one (neuter)

You could think of sú here as replacing íbúðin or íbúð.

Because the relative clause is describing the apartment by a fact that is presented as currently true or generally true:

• sú sem kostar níutíu þúsund = the one that costs ninety thousand

This is natural in Icelandic, just as English can say:

• She looked at two apartments yesterday, but neither was as good as the one that costs ninety thousand.

If the speaker wanted to emphasize that the price was only true at that past time, kostaði could also be possible in a different context. But kostar is very normal here.

It is the amount governed by kosta (to cost), which normally takes an accusative object.

So the apartment:

• kostar níutíu þúsund = costs ninety thousand

In this expression, the number phrase does not show much visible case change, so learners often do not notice the case directly. But syntactically, it functions as the object of kostar.

En means but.

It joins two clauses:

• Hún skoðaði tvær íbúðir í gær = She looked at two apartments yesterday
• en hvorug þeirra var jafn góð... = but neither of them was as good...

So en marks a contrast between the two ideas.