Learn Hardcore Icelandic: Hún gerir sömu æfingu á hverjum degi, því það er orðin venja. - She does the same exercise every day, because it has become a habit.

Hún – she (nominative singular, 3rd person, feminine)
gerir – does / makes (3rd person singular present of gera)
sömu – same (feminine accusative singular of sami)
æfingu – exercise (accusative singular of æfing)
á – on / in / every (preposition; here it introduces a time phrase)
hverjum – each / every (dative masculine singular of hver)
degi – day (dative singular of dagur)
því – because (here used as a conjunction meaning because)
það – it (neuter singular pronoun; here a dummy/placeholder subject)
er – is (3rd person singular present of vera “to be”)
orðin – past participle of verða “to become”; feminine nominative singular
venja – habit (nominative singular, feminine)

Natural English: She does the same exercise every day because it has become a habit.

Why is it sömu æfingu and not sama æfingu?

Sami (“same”) is irregular and changes form depending on gender and case.

For feminine singular:

Nominative: sama
Accusative: sömu
Dative: sömu
Genitive: sömu

In the sentence, æfingu is feminine accusative singular (direct object of gerir), so sami must also be feminine accusative singular: sömu æfingu.

If it were nominative (subject), it would be sama æfing. Here it’s object, so sömu æfingu is the correct form.

Why does æfing become æfingu here? What case is that?

Æfing is a feminine noun. Its singular forms include:

Nominative: æfing
Accusative: æfingu
Dative: æfingu
Genitive: æfingar

In Hún gerir sömu æfingu, the noun is the direct object of gera (“to do”), so it must be in the accusative.

That’s why you see æfingu (accusative singular feminine) instead of æfing.

Why is it á hverjum degi and not something like á hvern dag?

There are actually two common ways to say every day:

á hverjum degi – literally “on each day”
- á takes the dative here (a location in time)
- hverjum is dative masculine singular of hver
- degi is dative singular of dagur
hvern dag – literally “each day”
- Here there is no preposition; the time expression stands alone in the accusative.

Both are correct and common:

Hún gerir sömu æfingu á hverjum degi.
Hún gerir sömu æfingu hvern dag.

The version in your sentence happens to use the preposition + dative pattern.

Why does á take the dative in á hverjum degi? I thought á could use accusative too.

The preposition á can take either accusative or dative; the choice depends on meaning:

Accusative with movement / direction:
- Hún setur bókina á borðið. – She puts the book onto the table.
Dative with location / position / time:
- Bókin er á borðinu. – The book is on the table.
- Á hverjum degi – On / every day (a point in time, no movement).

In á hverjum degi, we are talking about the time when something happens (no motion), so á governs the dative. Hence hverjum degi, not hvern dag under á.

What exactly is því doing here? Isn’t af því að the usual “because”?

Því has several related uses:

As part of the causal conjunction af því að = because
As því að, also meaning because
By itself, því can function like a conjunction or sentence connector meaning because / for / as or therefore, depending on context.

In your sentence:

..., því það er orðin venja.

því is functioning as “because”. A slightly more explicit version could be:

..., af því að það er orðin venja.
..., því að það er orðin venja.

Using just því here is quite natural and common in Icelandic writing and speech.

Why is it það er orðin venja instead of something like það hefur orðið venja for “it has become a habit”?

Icelandic often expresses a resulting state with:

vera (to be) + past participle

rather than with hafa + past participle (the typical perfect form).

það er orðin venja
- literally: “it is become habit”
- idiomatically: “it has become a habit (and now is one)”

If you said:

það hefur orðið venja

that would be understood, but it sounds more like focusing on the event of becoming, not the current state. In this context, Icelandic strongly prefers er orðin to highlight that it now is a habit as a result of past repetition.

Why is the participle orðin (feminine) and not orðið (neuter) even though það is neuter?

The past participle of verða (“to become”) is:

Masculine nominative singular: orðinn
Feminine nominative singular: orðin
Neuter nominative singular: orðið

In það er orðin venja, the agreement is with venja, not with það:

venja is feminine nominative singular
therefore the participle must be orðin (feminine)

Syntactically, það here is a dummy/expletive subject, and venja is the predicative noun that carries gender; agreement follows venja:

Það er orðin venja. – It has become a habit.
Compare:
- Hann er orðinn læknir. – He has become a doctor. (masc orðinn)
- Hún er orðin læknir. – She has become a doctor. (fem orðin)
- Það er orðið kalt. – It has become cold. (neut orðið, agreeing with kalt)

So the form of orð‑ changes to match the gender of the thing that has become something (here: venja, feminine).

Could I drop það and just say ..., því er orðin venja?

No, that would be ungrammatical. You need some kind of subject or dummy subject.

Acceptable options:

..., því það er orðin venja.
..., af því að það er orðin venja.
..., því að það er orðin venja.

But *því er orðin venja (with no það) is not correct modern Icelandic. The það is the placeholder subject required by the verb er.

Is gera æfingu the normal way to say “do an exercise / work out”? Could I use a verb instead of the noun?

Yes, gera æfingu is a very normal and idiomatic way to talk about doing a physical or practice exercise:

gera æfingu / æfingar – do an exercise / do exercises
gera jógaæfingar – do yoga exercises
gera styrktaræfingar – do strength exercises

There is also the verb æfa, meaning “to exercise / to practise”:

Hún æfir á hverjum degi. – She exercises/practises every day.
Hún æfir jóga á hverjum degi. – She practises yoga every day.

So you could say:

Hún æfir á hverjum degi, því það er orðin venja.

but the original Hún gerir sömu æfingu á hverjum degi is perfectly natural and maybe a bit more specific (one particular exercise).

Can I change the word order and say Hún gerir á hverjum degi sömu æfingu?

Yes, Hún gerir á hverjum degi sömu æfingu is grammatically correct.

Word order in Icelandic is fairly flexible for adverbials like time phrases. The neutral, most common order here is:

Hún gerir sömu æfingu á hverjum degi.

Moving á hverjum degi earlier (Hún gerir á hverjum degi sömu æfingu) can give a bit more emphasis to the frequency (“every day”) rather than to the specific exercise, but in everyday speech the difference is subtle. Both are fine.

Could I say Hún gerir sömu æfinguna á hverjum degi instead? What’s the difference?

Yes:

Hún gerir sömu æfinguna á hverjum degi.

is also correct. The difference is definiteness:

sömu æfingu – the same exercise in a generic / indefinite sense
- could mean “the same kind of exercise” or “the same one each time” depending on context.
sömu æfinguna – the same exercise but definite
- strongly suggests a specific, already known exercise that both speaker and listener have in mind.

English often doesn’t mark this difference clearly, but Icelandic does it with the definite ending -una on æfinguna.