Learn Hardcore Icelandic: Eftir stutta útivist í skóginum er hún alltaf mjög hress. - After a short outdoor activity in the forest she is always very energetic.

útivist is a feminine noun.
The phrase eftir stutta útivist uses útivist in the accusative singular (because of the preposition eftir, see below).
An adjective must agree with the noun it describes. So the adjective stuttur (short) becomes:
- Nominative feminine singular: stutt
- Accusative feminine singular: stutta

So:

stuttur = masc. nom. sg. (e.g. stuttur dagur – a short day)
stutt = fem. nom. sg. (e.g. stutt útivist – short outdoor activity)
stutta = fem. acc. sg. (e.g. eftir stutta útivist – after a short outdoor activity)

Because of eftir + accusative, you need the accusative form, so: stutta útivist.

Which case does eftir take here, and why is útivist in that case?

In this sentence, eftir governs the accusative case:

eftir stutta útivist → útivist is in the accusative singular, and the adjective stutta agrees with that.

eftir can take different cases depending on meaning:

Accusative when it has a more concrete / time-related meaning:
- eftir stutta útivist – after a short time outdoors
- eftir tvo daga – after two days
Dative in some more abstract or idiomatic uses (e.g. eftir honum ≈ after him / following him), but for learning purposes, you can safely remember:
- Time-related “after X” → accusative

So in this sentence, eftir = after (a period of time), which triggers the accusative.

What exactly does útivist mean, and is it countable like “an outdoor activity”?

útivist is a feminine noun that literally means being outdoors / outdoor recreation. It often refers to:

spending time outside, often in nature
activities like hiking, walking, skiing, etc.

Important points:

It’s usually treated as a mass/abstract noun in Icelandic, more like “outdoor activity / time outdoors” in general, not “one outdoor activity, two outdoor activities”.
In this sentence, stutta útivist is best understood as “a short time spent outdoors” or “a bit of outdoor activity”, not “one outing” in a strict countable sense.

Grammatically:

Gender: feminine
Nominative singular: útivist
Here: accusative singular (same form in this case), modified by stutta.

Why is it í skóginum and not í skógurinn or just í skógi?

Several things are happening in í skóginum:

Case:
- The preposition í can take accusative (movement into) or dative (location in).
- Here it means “in the forest” (location), so it takes the dative.
- Dative singular of skógur (a forest) is skógi.
Definiteness:
- Icelandic often uses the definite article (the -inn suffix) where English also says “the”.
- skógur = a forest
- skógurinn = the forest (nominative)
- In the dative definite, it becomes skóginum.

So:

í skógi = in a forest
í skóginum = in the forest

The sentence specifically has “in the forest”, hence í skóginum: preposition í + dative skógi + definite ending -num → skóginum.

Why does the sentence start with Eftir stutta útivist í skóginum and then put the verb er before hún? Why not … hún er alltaf … like in English?

Icelandic (like German) has a verb-second (V2) rule in main clauses:

The finite verb (here er) must be the second element in the clause.
The first element can be the subject, an adverb, a prepositional phrase, etc.

In the neutral order, you could say:

Hún er alltaf mjög hress eftir stutta útivist í skóginum.
(Subject first → Hún; verb second → er.)

But if you want to emphasize the time phrase “after a short time outdoors in the forest,” you can move it to the front. Then, to obey V2:

1st element: Eftir stutta útivist í skóginum
2nd element: er (the verb)
3rd element: hún (the subject)

So: Eftir stutta útivist í skóginum er hún alltaf mjög hress.

This inversion (verb before subject) is normal and required in Icelandic whenever something other than the subject comes first in a main clause.

What does hress really mean here? “Healthy”? “Happy”? “Energetic”?

hress is a very common Icelandic adjective with a meaning that overlaps several English words:

feeling fresh, energetic, in good spirits, cheerful, often also implying physically well / refreshed.

In this sentence, after outdoor activity, mjög hress suggests:

she feels very refreshed / energized / perky / in a good mood.

Depending on context, hress can be translated as:

Heilbrigður / healthy (in some contexts, but that’s not its core sense)
Cheerful, upbeat, lively, in good spirits
Feeling well / back to normal after being ill or tired

Here, the natural English idea is: “she always feels very refreshed / energized”.

Is hress inflected here? Why doesn’t it change form for hún?

hress is an adjective, and it does decline, but in this sentence:

It’s used in the predicative position: hún er hress (“she is hress”).
For many adjectives, the predicative form for masculine/feminine singular can look the same, especially in the indefinite form.

Base forms of hress:

Masculine nominative sg.: hress
Feminine nominative sg.: hress
Neuter nominative/accusative sg.: hresst

Because hún is feminine, nominative singular, hress is the correct form and it happens to look the same as the masculine in this particular case.

So the adjective is agreeing with hún in gender, number and case; it just doesn’t visibly change shape here.

Where do alltaf and mjög go in the sentence? Could you say them in other positions?

In the sentence:

Eftir stutta útivist í skóginum er hún alltaf mjög hress.

we have:

alltaf (always) – an adverb of frequency
mjög (very) – an adverb of degree, modifying hress

The neutral and natural order is:

er hún alltaf mjög hress
→ [verb] [subject] [frequency adverb] [degree adverb] [adjective]

You cannot move mjög away from hress, because it specifically modifies that adjective:

✗ … er hún alltaf hress mjög – unnatural
✓ … er hún alltaf mjög hress – natural

You can sometimes move alltaf a bit more freely, but there are limits. For example:

Hún er alltaf mjög hress. (neutral)
Alltaf er hún mjög hress. (emphasizes “always”; still grammatical)

In the original sentence with the fronted time phrase, the order er hún alltaf mjög hress is the most natural and common.

Could you also say Eftir stutta útivist í skóginum, þá er hún alltaf mjög hress?

You can hear sentences like that in spoken Icelandic, but:

The þá (“then”) is often considered redundant or a bit colloquial when the time phrase is already clear.
Written standard style usually omits þá in this structure.

So:

Eftir stutta útivist í skóginum er hún alltaf mjög hress.
→ clean, standard.
Eftir stutta útivist í skóginum, þá er hún alltaf mjög hress.
→ understandable, somewhat more spoken / informal; þá adds a “then” that English often doesn’t translate either.

For learning purposes, the cleaner version without þá is better to imitate.

Could I replace útivist with something more specific, like “walk”, and keep the structure the same?

Yes. You can plug in other nouns while keeping the same grammar:

Eftir stutta göngu í skóginum er hún alltaf mjög hress.
- ganga (a walk, hike) – feminine noun
- Here: stutta göngu (fem. acc. sg.) agreeing with göngu.
Eftir stutt hlaup í skóginum er hún alltaf mjög hress.
- hlaup (a run) – neuter noun
- stutt hlaup (neut. acc. sg.) – no extra -a here because neuter singular often ends in -t without an added vowel.

The pattern to remember:

Eftir
- [accusative phrase describing the activity] + í skóginum (dative, location) + [V2 clause].

Is there a version where útivist is in another case, like dative, and what would that change in meaning?

Yes, in other contexts you can see útivist in other cases. For example:

Hún er í útivist. – She is in outdoor activities / in P.E. (school subject).
- í here takes dative → útivist in the dative singular (same form in this case).
Hún nýtur útivistar. – She enjoys outdoor life / activity.
- nýta often takes genitive → útivistar (genitive singular).

But in our sentence, because of the time meaning “after X”, eftir uses the accusative, so:

Eftir stutta útivist → accusative singular.

Changing the case here (e.g. making it dative) would simply be wrong grammar in this particular structure and wouldn’t express a new nuance; it would just be incorrect with eftir in the “after (some time of) X” meaning.