Ég er ekki hissa á því að hún sé þreytt; hún hefur verið að vinna síðan klukkan sex.

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

So:

• Ég er ekki hissa = I am not surprised
• literally: I am not surprised

This is very normal Icelandic word order. An English speaker may want to put ekki somewhere else, but after the finite verb is the standard pattern in simple main clauses like this.

This is an idiomatic pattern:

• vera hissa á e-u / e-m = to be surprised at something / someone
• vera hissa á því að + clause = to be surprised that ...

So Ég er ekki hissa á því að hún sé þreytt means literally something like:

• I am not surprised at it that she be tired

But in natural English, that is simply:

• I am not surprised that she is tired

The important thing for learners is that hissa commonly goes with á.

Because hissa normally takes the preposition á, and that preposition needs an object. Here the object is því, the dative form of það.

So the structure is:

• á því að ...

You can think of it as something like at the fact that ...

Grammatically:

• það = it / that
• því = dative form of það

The dative is used because of the preposition á in this expression.

Hissa is an indeclinable adjective, which means it does not change form for gender, number, or case.

So you get:

• ég er hissa
• hann er hissa
• hún er hissa
• við erum hissa

That is different from many other Icelandic adjectives, which do change form.

Sé is the present subjunctive of vera.

• er = present indicative
• sé = present subjunctive

In subordinate clauses after certain expressions of reaction, judgment, report, or attitude, Icelandic often uses the subjunctive. Here the speaker is reacting to the situation, so sé is very natural.

So:

• að hún sé þreytt = that she is tired

English usually does not show this difference clearly, but Icelandic does.

Because þreyttur is a normal adjective that does agree with the subject.

Its nominative singular forms are:

• masculine: þreyttur
• feminine: þreytt
• neuter: þreytt

Since hún is feminine singular, the correct form here is þreytt.

So:

• hann er þreyttur
• hún er þreytt
• barnið er þreytt

No. They have different jobs.

1. að in á því að hún sé þreytt

   • this is a conjunction
   • it means that

2. að in að vinna

   • this is the infinitive marker
   • it is the to of to work

So even though both words are spelled að, they are doing different things in the sentence.

This is a very common Icelandic way to express an ongoing action up to the present, much like English has been working.

Breakdown:

• hefur = has
• verið = past participle of vera
• að vinna = working / to work

So:

• hún hefur verið að vinna = she has been working

This comes from the construction:

• vera að + infinitive = be doing something

So:

• hún er að vinna = she is working
• hún hefur verið að vinna = she has been working

You could say hún hefur unnið, but it is not exactly the same.

• hún hefur unnið = she has worked / she has done work
• hún hefur verið að vinna = she has been working

The version with verið að emphasizes the ongoing activity and fits especially well with the idea that she is tired now because the work has been continuing since a specific time.

Síðan here means since.

So:

• síðan klukkan sex = since six o’clock

The word klukkan is commonly used when telling clock time:

• klukkan sex = at six o’clock / six o’clock

So in this sentence:

• hún hefur verið að vinna síðan klukkan sex
• she has been working since six o’clock

Also note that síðan can mean other things in other contexts, such as then or after that, but here it clearly means since.

The semicolon links two closely connected independent clauses:

• Ég er ekki hissa á því að hún sé þreytt
• hún hefur verið að vinna síðan klukkan sex

The second clause explains the first one, so a semicolon works well. A full stop would also be possible. The semicolon just shows a strong connection between the two ideas.