Learn Hardcore Icelandic: Stutt æfing á morgnana er góð venja. - A short exercise in the mornings is a good habit.

The base adjective is stuttur = short (masculine, nominative singular, strong declension).
The noun æfing (exercise / workout) is feminine, singular, nominative.

For a feminine, nominative singular noun, the strong form of stuttur is stutt:

masculine: stuttur maður – a short man
feminine: stutt æfing – a short exercise
neuter: stutt bréf – a short letter

So stutt is the correctly inflected form to match æfing.

What gender and case is æfing, and what does it mean here?

æfing is a feminine noun.
In this sentence it is in the nominative singular: æfing (not æfingu, æfingar, etc.).
It is the subject of the sentence: Stutt æfing á morgnana = A short workout in the mornings.

Meaning-wise, æfing can mean:

exercise, workout (physical exercise)
practice, drill, exercise (for skills, music, sports, etc.)

Here, from context, it’s best understood as a short workout / short exercise session.

What exactly does á morgnana mean, and why that particular ending?

á morgnana means “in the mornings” / “on (the) mornings” in the sense of a repeated, habitual time.

Breakdown:

á – preposition, here meaning on / in (time)
morgunn – morning (masculine noun)
morgnana – this is accusative plural, definite of morgunn:
- nominative plural definite: morgnarnir – the mornings
- accusative plural definite: morgnana – (onto / on) the mornings

So literally, á morgnana is “on the mornings”, but idiomatically it is “in the mornings” (as a general habit).

This accusative plural definite with á is common in habitual time expressions:

á kvöldin – in the evenings
á daginn – in the daytime
á laugardagana – on Saturdays (habitually)

So á morgnana is the standard idiomatic way to say habitually in the mornings.

Could I also say á morgnunum instead of á morgnana?

á morgnunum is dative plural definite (on the mornings), and it is grammatically possible, but:

For the meaning “in the mornings” as a general habit, the natural, idiomatic form is á morgnana.
á morgnunum can sound more like referring to some specific set of mornings already known from context, or simply less idiomatic in this generic “good habit” statement.

So for a general rule or habit, use:

✅ Stutt æfing á morgnana er góð venja.

Why is there no word for “a” in Stutt æfing á morgnana?

Icelandic does not have an indefinite article like English “a / an”.

æfing on its own can mean an exercise / a workout or just exercise depending on context.
Indefiniteness is expressed simply by using the bare noun without the definite article.

So:

Stutt æfing á morgnana
= Short exercise in the mornings / A short workout in the mornings

There is no dedicated word for “a”; Icelandic just doesn’t use one.

Why is the verb er (singular “is”) used here, when there are two nouns?

The structure is:

Subject: Stutt æfing á morgnana – A short workout in the mornings
Verb: er – is
Predicate (complement): góð venja – a good habit

Even though there are two nouns (æfing and venja), they are not both subjects. The whole phrase “Stutt æfing á morgnana” is one subject phrase, and “góð venja” is what it is being equated with.

Therefore the verb agrees with the single subject in the third person singular:

… er góð venja – … is a good habit

Why is it góð venja and not góður venja?

Again, this is adjective agreement:

venja (habit) is a feminine noun.
The adjective góður (good) must match it in gender, number, and case.

For feminine nominative singular:

masculine: góður maður – a good man
feminine: góð venja – a good habit
neuter: gott barn – a good child

So góð venja is the correct form.

What is the difference between venja and vani, both meaning “habit”?

Both can translate as “habit”, but there is a nuance:

venja
- A practice, routine, custom you have or cultivate.
- Slightly more “neutral” or “chosen”: something you do regularly, often on purpose.
- Very natural in general advice sentences:
  - Að lesa á kvöldin er góð venja. – Reading in the evenings is a good habit.
vani
- More like a habit / tendency / routine that characterizes someone.
- Can be neutral or slightly negative, like something you’ve grown used to, not necessarily intentionally chosen.
- E.g.: Hann hefur þann vana að koma seint. – He has the habit of coming late.

In this sentence, góð venja is the more natural choice, because we’re talking about a recommended routine.

Can the word order be changed, for example Á morgnana er stutt æfing góð venja?

Icelandic word order is fairly flexible, but not all possible orders sound equally natural.

The original:

Stutt æfing á morgnana er góð venja.
– Very natural, subject first, then verb, then complement.

You can also front the time expression for emphasis on when:

Á morgnana er stutt æfing góð venja.
– Grammatically possible; sounds like you’re especially highlighting “In the mornings…”.
– Still acceptable, though the original is more straightforward and typical.

Another natural variant is to move only the time phrase:

Á morgnana er stutt æfing góð venja fyrir marga.
– In the mornings, a short workout is a good habit for many.

So yes, you can change the order, but the given sentence is already very natural.

Could I say something like á morgninum instead of á morgnana? What would that mean?

á morgninum is dative singular definite: on the morning / in the morning (a specific one).

á morgnana – in the mornings (habitually, in general)
á morgninum – in the morning (usually a particular morning or a specific time of day in a specific context)

For a general rule or habit, Icelandic strongly prefers the plural, habitual form:

✅ Stutt æfing á morgnana er góð venja. – A short workout in the mornings is a good habit.

You’d use á morgninum more if you were talking about a particular day / time, e.g.:

Á morgninum er oft erfitt að vakna. – In the morning it is often hard to wake up. (referring e.g. to today/most days, but as a single part of the day)

Is there a more natural or more typical way to say this in everyday Icelandic?

Your sentence is already correct and natural.

In more casual or slightly expanded speech, someone might say, for example:

Það er góð venja að taka stutta æfingu á morgnana.
– It’s a good habit to do a short workout in the mornings.
Það er góð venja að hreyfa sig aðeins á morgnana.
– It’s a good habit to move a bit in the mornings.

But as a simple, clear statement, Stutt æfing á morgnana er góð venja. is perfectly idiomatic.