Learn Hardcore Swedish: I dag springer hon bara hälften så långt som igår. - Today she runs only half as far as yesterday.

Some element comes first (subject, time, place, etc.). Here it’s I dag.
The finite verb must be the second element in the sentence. Here that’s springer.
The subject usually comes next. Here that’s hon.

So the structure is:

I dag (today – first position)
springer (runs – finite verb, second position)
hon (she – subject, third slot)
bara hälften så långt som igår (the rest of the sentence)

If you started with the subject instead, you’d get:

Hon springer bara hälften så långt som igår i dag. (grammatical but the i dag at the end is less natural here)

The crucial point: in a main clause, the finite verb must be in second position, no matter what comes first.

Why is it I dag and not idag? Are both correct?

Both I dag and idag are correct in modern Swedish.

I dag (two words) is the more traditional and slightly more formal spelling.
idag (one word) is very common in everyday writing and is fully accepted.

You’ll see both in newspapers, books, and online. The same pattern exists for:

i går / igår (yesterday)
i morgon / imorgon (tomorrow)

Your sentence could also be written as:

Idag springer hon bara hälften så långt som igår.

No change in meaning.

Why is it igår and not something like på igår or i igår?

In Swedish, igår (or i går) already includes the preposition i (in). Historically, it was i går = in yesterday (the day that is gone), which became a set expression.

You say igår (or i går) by itself:
- Hon sprang långt igår. – She ran far yesterday.

You do not add another preposition like på or another i in front of it in this kind of sentence. So:

✅ som igår = as (far) as yesterday
❌ som på igår, som i igår

Why is the verb in the present tense (springer) when we’re comparing with something that happened yesterday?

Springer is in the present tense because it describes what she is doing today:

I dag springer hon … – Today she runs …

The part about yesterday is just a reference point for comparison:

… bara hälften så långt som igår. – … only half as far as (she did) yesterday.

We don’t need to repeat the past tense verb for yesterday; it’s understood:

som igår (då sprang hon längre)
= as yesterday (then she ran farther)

So the tenses are:

Present for the action happening today (springer).
Implied past for yesterday inside the comparison (som igår).

What does the structure hälften så långt som literally mean, and how does it work?

Hälften så långt som literally corresponds to English “half as far as”.

It’s built from:

hälften = the half
så … som = as … as (a standard comparison pattern)
långt = far (adverb form from the adjective lång = long)

So:

hälften så långt som igår
= half as far as yesterday (was / she ran yesterday)

The pattern så [adjective/adverb] som is very common:

så lång som – as tall/long as
så snabbt som – as fast as
så ofta som – as often as

Here hälften modifies that comparison: half as X as … → hälften så långt som ….

Why is som used in så långt som igår instead of än?

In Swedish, som and än are both used in comparisons, but in different patterns:

som is used in equality/comparison patterns like så … som:
- så lång som – as tall as
- lika dyr som – as expensive as
- hälften så långt som – half as far as
än is used with comparatives like större, längre, mer:
- längre än igår – farther than yesterday
- dyrare än förra året – more expensive than last year

So you say:

✅ hälften så långt som igår – half as far as yesterday
✅ kortare än igår – shorter/less far than yesterday
❌ hälften så långt än igår – ungrammatical

Why is it långt and not lång in så långt?

Långt here is an adverb, not an adjective.

lång (without -t) is the adjective form:
- en lång väg – a long road
- en lång bok – a long book
långt (with -t) is the adverb form meaning far:
- Hon springer långt. – She runs far.
- Hur långt springer hon? – How far does she run?

In hälften så långt som igår, långt describes how far she runs, not what something is like as a noun, so the adverb form is required.

What exactly does bara do here, and why is it in that position?

Bara means “only / just” here and limits the amount:

springer hon bara hälften så långt
= she runs only half as far

About the position:

Neutral word order in Swedish (after the V2 part) is:

Verb (already in second position) – springer
Subject – hon
Sentence adverbials (bara, inte, aldrig, etc.)
The rest – hälften så långt som igår

So:

I dag springer hon bara hälften så långt som igår.

If you move bara, you change what is being limited or emphasised:

I dag bara springer hon hälften så långt som igår.
→ sounds like It’s only today that she runs half as far, with focus on today, not on the distance. Unusual in isolation.

The given sentence is the normal, neutral placement of bara for “only half as far”.

Could I replace bara with endast in this sentence?

Yes, you can:

I dag springer hon endast hälften så långt som igår.

Both bara and endast mean “only”, but:

bara is more colloquial and neutral in everyday speech and writing.
endast feels more formal, written, or stylistically elevated—common in official texts, instructions, etc.

Meaning-wise, they’re the same here, just with a different tone.

Why is it som igår and not something longer like som hon sprang igår?

Swedish often leaves out information that is obvious from context. The full idea behind:

hälften så långt som igår

is something like:

hälften så långt som (hon sprang) igår
= half as far as (she ran) yesterday.

Since yesterday already clearly refers to her run yesterday, repeating hon sprang would be redundant. So Swedish allows ellipsis (omission) here, and som igår is completely natural and idiomatic.

You can say a longer version if you want to be extra explicit:

I dag springer hon bara hälften så långt som hon sprang i går.
(Perfectly grammatical, just a bit heavier.)

Can I change the word order and say Hon springer bara hälften så långt som igår i dag?

It’s grammatical, but it sounds less natural and a bit clumsy.

Possible variants:

I dag springer hon bara hälften så långt som igår.
– Most natural, with a clear time frame at the start.
Hon springer bara hälften så långt som igår i dag.
– Grammatically okay, but putting i dag at the end can make the sentence feel “heavy” and slightly confusing until you reach the final i dag.
Hon springer bara hälften så långt som igår.
– Now it’s ambiguous: it tends to be interpreted as a general statement (She only runs half as far as yesterday [usually]), because you’ve lost the clear “today” anchor.

If you want to keep the clear “today vs yesterday” contrast, starting with I dag is the most straightforward: I dag springer hon bara hälften så långt som igår.

Is there another way to say this, more literally like “half of the distance she ran yesterday”?

Yes, you can express it more literally with hälften av:

I dag springer hon bara hälften av sträckan hon sprang i går.
= Today she runs only half of the distance she ran yesterday.

Differences:

hälften så långt som igår
– Uses a comparative construction, very idiomatic and concise.
hälften av sträckan hon sprang i går
– More explicit, slightly more formal or written-sounding.

Both are correct; your original sentence with hälften så långt som igår is perfectly natural Swedish.