Learn Hardcore Icelandic: Bak mitt er þreytt eftir langan dag í vinnu. - My back is tired after a long day at work.

If you put mitt before the noun (mitt bak), it can sound either emphatic, poetic, or sometimes unnatural, depending on context. For ordinary, neutral speech about body parts, noun + possessive is the regular pattern:

hönd mín = my hand
fótur minn = my leg / foot
augað mitt = my eye

So bak mitt is the normal, everyday way to say my back here.

Why is it bak and not bakið (with the definite article), like the back?

Icelandic uses the definite form (with -ið, -inn, -in etc.) differently from English. For body parts, when you add a possessive pronoun (mitt, þitt, hans, hennar, etc.), you usually do not also mark the noun as definite.

bak = (a) back
bakið = the back
bak mitt = my back (literally: back my)

English has to say my back, not the back, so we feel tempted to make it the back my in Icelandic. But Icelandic thinks of it as already specific because of mitt, so it doesn’t need -ið as well.

You could say bakið mitt in some special contexts (e.g. contrasting the back as a specific part vs some other back), but the neutral way in this sentence is simply bak mitt.

Why is the adjective þreytt and not þreyttur or some other form?

Adjectives in Icelandic agree with the noun in gender, number, and case.

The basic adjective is: þreyttur (tired) – masculine nominative singular.
Neuter nominative singular is: þreytt.

The subject here is bak mitt:

bak is a neuter noun.
It’s the subject, so it’s in the nominative case, singular.

Therefore, the adjective must also be:

þreytt = neuter, nominative, singular

So:

Bak mitt er þreytt. = My back is tired.

Compare:

Ég er þreyttur. (a man speaking)
Ég er þreytt. (a woman speaking – feminine form)
Barnið er þreytt. (the child is tired – child is neuter, so þreytt)

What does eftir do here, and which case does it take?

In this sentence, eftir means after in a temporal sense:

eftir langan dag = after a long day

With this time-meaning, eftir typically takes the accusative case. That’s why:

dagur (day, nominative) → dag (accusative)
The adjective langur (long) in masculine accusative singular becomes langan.

So:

eftir langan dag = after a long day (accusative phrase)

eftir can have other meanings (e.g. to look after, according to, etc.), and its case usage can vary with meaning, but for after (a period of time), accusative is standard.

Why do we say langan dag and not just langur dagur?

The basic combination is:

langur dagur = a long day (nominative: used for subjects)

But here, eftir (after) is followed by a time expression in the accusative case. So both the noun and the adjective must shift to the accusative:

Noun: dagur (nom.) → dag (acc.)
Adjective (masc. sing.):
- nominative: langur
- accusative: langan

So we get:

eftir langan dag = after a long day

This shows typical Icelandic agreement: the adjective changes form to match the case, gender, and number of the noun it describes.

Why is it í vinnu and not í vinnuna or í vinnunni?

The preposition í can take either accusative or dative, depending on the meaning:

í
- accusative → motion into something (into)
í
- dative → location in something (in)

Here, í vinnu means at work / in (the state of) working, not movement into work. It’s more like a general state or activity.

vinna (work, job) – feminine noun
Dative singular of vinna is vinnu

So:

í vinnu = in work / at work (dative, no definite article)

Compare:

Ég er í vinnu. = I am at work.
Ég fer í vinnuna. = I am going to work. (accusative, definite: vinnuna)
Ég er í vinnunni. = I am at the (specific) workplace. (dative + definite: vinnunni)

In your sentence, í vinnu is the usual, neutral way to say at work in a general sense.

Is the word order fixed, or could I say something like Bak mitt, eftir langan dag í vinnu, er þreytt?

The normal, neutral word order is:

Bak mitt er þreytt eftir langan dag í vinnu.

This is Subject – Verb – Predicate – Adverbial (time phrase).

You can move the time phrase around for emphasis or style:

Eftir langan dag í vinnu er bak mitt þreytt.
(Emphasis on “after a long day at work”.)

You could also insert the phrase with commas:

Bak mitt, eftir langan dag í vinnu, er þreytt.

That sounds more written or stylistic, like a slight pause in speech. It is grammatically possible, but for learners, it’s best to stick to the basic word order first:

Subject + er + adjective + time phrase
Bak mitt er þreytt eftir langan dag í vinnu.

Are there other natural ways to say “My back is tired after a long day at work” in Icelandic?

Yes, several alternatives are common and natural. For example:

Ég er þreytt(ur) í bakinu eftir langan dag í vinnu.
- Literally: I am tired in the back after a long day at work.
- Very idiomatic; many speakers might prefer this.
Ég er svo þreytt(ur) í bakinu eftir vinnu.
- I’m so tired in my back after work.
To talk about pain instead of just tiredness:
- Mig verkjar í bakinu eftir langan dag í vinnu.
  - My back aches after a long day at work. (literally: It aches me in the back…)

Your original sentence Bak mitt er þreytt… is correct and understandable, though with body parts Icelandic quite often uses structures like Ég er X í bakinu or Mig verkjar í bakinu.

How do you pronounce þreytt and what sound is þ?

þ in Icelandic is pronounced like th in English thin or thing (a voiceless “th” sound).
So þreytt is roughly like: THRAYTT (one syllable).

More precisely:

þr- = like thr in English three
ey = a diphthong, similar to English ay in day
tt at the end is a short, hard t sound, often with a slight glottal stop feel for English speakers.

So you can approximate:

þreytt ≈ [θreɪht] or THRAYT

The Ð/ð letter, by contrast, is the voiced “th” (like English this), but it doesn’t appear in this sentence.

What is the difference between vinna and vinnu in terms of pronunciation and grammar?

Grammar:

vinna = the base (nominative singular) form of the noun work / job (feminine).
vinnu = the accusative or dative singular form of the same noun.

In í vinnu, it’s dative singular because í here expresses location/state (“in work / at work”).

Pronunciation:

vinna ≈ VIN-na (two syllables)
- i like short i in sit
- nn is long (double)
- final -a like a short a in sofa (unstressed)
vinnu ≈ VIN-nu
- again short i
- nn long
- u is a short, rounded vowel (somewhere between English uh and oo)

Rhythmically: VIN-na vs VIN-nu – the difference is mainly in the final vowel.