Learn Hardcore Icelandic: Slysin á veturna eru oft verri þegar vegurinn er sleipur. - The accidents in the winter are often worse when the road is slippery.

Slysin á veturna: slightly more like “the accidents that happen in winter” as a known/general class; this is very idiomatic for such a general statement.
Slys á veturna: feels a bit more bare, like just mentioning accidents in winter without grouping them as a “type”.

In practice, both can be used generically, but the version with the definite (slysin) is more natural in this kind of statement.

What exactly does á veturna mean, and why is it á and not í?

Á veturna is a very common time expression meaning roughly:

in winter / during the winter / in the winters (as a rule)

The preposition á is widely used in Icelandic for repeated / habitual times, especially:

á morgnana – in the mornings
á kvöldin – in the evenings
á sumrin – in (the) summers
á veturna – in (the) winters

You could see í vetur or í veturinn, but those usually refer to a specific winter or this coming / last winter, not to winters in general.

So á veturna is the standard way to say “in winter (habitually, generally)”.

Why is it veturna and not just vetur in á veturna?

Vetur is a masculine noun:

Nom. sg.: vetur – winter
Acc. pl.: vetur – winters
Acc. pl. definite: veturna – the winters

The form veturna is:

plural
accusative
definite (ending -na)

So literally á veturna = “on the winters”, but idiomatically = “in (the) winters, in wintertime (generally)”.

The -na is the regular definite plural ending for masculine nouns in the accusative plural:

bílar → bílana (the cars, acc. pl.)
vetur → veturna (the winters, acc. pl.)

Why does á take the accusative in á veturna instead of the dative (á veturnum)?

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

á + accusative: movement onto / over, or “the whole period, general / habitual time”
á + dative: location on, or “at a specific time”

With time expressions, á + accusative is standard for habitual / general times:

á morgnana – in the mornings (generally)
á sumrin – in the summers (generally)
á veturna – in the winters (generally)

You might encounter á veturnum with the dative in some contexts, but the everyday idiom for a general statement like this is á veturna (accusative).

What is the subject of the sentence? Is á veturna part of the subject?

The grammatical subject is slysin.

Slysin – subject (nom. pl.)
á veturna – adverbial phrase of time (when)
eru – verb (are)
oft verri – predicate / complement (often worse)
þegar vegurinn er sleipur – subordinate time clause (when the road is slippery)

So the subject is just “the accidents”, and á veturna tells you when.

Why is the verb eru and not er?

Eru is the 3rd person plural form of vera (to be).

3rd person singular: hann / hún / það er – he / she / it is
3rd person plural: þeir / þær / þau eru – they are

Here the subject slysin is plural (accidents), so the verb must also be plural:

Slysin … eru oft verri – The accidents … are often worse.

Where does the form verri come from, and what is the base adjective?

Verri is the comparative form of “bad”.

In Icelandic there are three common adjectives meaning bad:

vondur, slæmur, illur

All of them share an irregular comparative:

Positive (base): vondur / slæmur / illur – bad
Comparative: verri – worse
Superlative: verstur (or short form verst) – worst

So verri doing the job of “worse” here:
Slysin … eru oft verri – Accidents … are often worse.

Why does verri end in -i even though slysin is neuter plural? Shouldn’t it look neuter?

Comparative adjectives in Icelandic decline differently from normal (positive) adjectives.

Positive adjectives like slæmur have gendered endings in the plural:

slæmir (m. pl. nom.), slæmar (f. pl. nom.), slæm (n. pl. nom./acc.)

But comparative forms like verri are declined weakly, and in the nominative plural they usually take -i for all genders:

m./f./n. nom. pl.: verri

So:

slysin eru verri – the accidents are worse
börnin eru yngri – the children are younger
húsin eru nýrri – the houses are newer

Even though slysin is neuter plural, the comparative form still shows up as verri with -i. That’s just how comparatives work in Icelandic.

Why is the word order eru oft verri and not eru verri oft or something else?

In a normal Icelandic main clause:

The verb tends to be in second position (V2 rule).
Common adverbs like oft usually come right after the finite verb.

So:

Subject: Slysin á veturna
Verb: eru
Adverb: oft
Predicate adjective: verri

→ Slysin á veturna eru oft verri …

You could move oft for emphasis in special contexts, but the neutral, natural order is to have oft immediately after the verb.

What does þegar mean here, and how is it different from ef or þá?

Þegar is a conjunction meaning when (time).

þegar vegurinn er sleipur – when the road is slippery

Contrast:

ef – if (condition):
- ef vegurinn er sleipur – if the road is slippery
þá – an adverb meaning then, often used in the main clause that follows a when / if clause:
- Þegar vegurinn er sleipur, þá eru slysin verri.
  – When the road is slippery, (then) the accidents are worse.

In your sentence, þegar simply introduces a time clause: it tells you under what circumstances (when) the accidents are worse.

Why is the word order þegar vegurinn er sleipur and not þegar er vegurinn sleipur?

In Icelandic:

Main clauses usually have verb‑second word order (V2):
- Vegurinn er sleipur. – The road is slippery.
- Í dag er vegurinn sleipur. – Today the road is slippery.
Subordinate clauses (introduced by þegar, að, ef, vegna þess að, etc.) do not have V2; the verb typically follows the subject:
- þegar vegurinn er sleipur – when the road is slippery
  - subject: vegurinn
  - verb: er

So the structure þegar vegurinn er sleipur is the normal subordinate clause order: conjunction – subject – verb – rest.

Why is it vegurinn er sleipur and not something like vegurinn er sleipur or sleipurinn?

A few points:

Vegurinn
- Base noun: vegur – road
- Nom. sg. definite: vegurinn – the road
  It’s masculine singular nominative, the subject of the clause.
Sleipur
This is the base adjective meaning slippery. Its strong nominative forms:
- m. sg.: sleipur
- f. sg.: sleip
- n. sg.: sleipt
After vera (to be), you generally use the indefinite (strong) form of the adjective, even when the noun is definite:
- maðurinn er stór – the man is big
- húsið er stórt – the house is big
- vegurinn er sleipur – the road is slippery
You don’t say vegurinn er sleipurinn in this neutral descriptive sense; the weak (definite) form of the adjective is used in other specific contexts (e.g. “the big one”, certain superlatives, etc.), not in simple X is ADJECTIVE statements like this.

Does the sentence mean that accidents are more frequent in winter, or that each accident tends to be more serious?

The Icelandic verri focuses on quality / severity, not frequency.

Slysin á veturna eru oft verri is understood as:
- Winter accidents are often *more serious / worse (e.g. injuries, damage) when the road is slippery.*

If you wanted to specifically say there are more accidents, you’d use something like:

Það verða fleiri slys á veturna. – There are more accidents in winter.

In your sentence, the idea is about how bad the accidents are, not how many occur.