Steinninn er á veginum.

They are steinn meaning stone and vegur meaning road.

In the sentence, you are seeing inflected forms:

• steinninn = the stone
• veginum = on the road with road in a different case form

If you wanted to look these words up in a dictionary, you would usually search for steinn and vegur.

Because Icelandic usually puts the definite article, English the, onto the end of the noun.

So:

• steinn = stone
• steinninn = the stone

This is one of the first big differences English speakers notice in Icelandic: there is often no separate word for the.

Because the base word already ends in -nn: steinn.

Then the definite article is added:

• steinn
  • definite ending = steinninn

So the spelling looks heavy with n's, but it is completely normal. It is not a typo.

In normal Icelandic, the is usually attached to the noun as an ending, not written as a separate word.

In this sentence:

• steinninn = the stone
• veginum = the road in dative case after á

Icelandic does have separate forms like hinn, hin, hið in some contexts, but that is not what is happening here.

Steinninn is in the nominative case because it is the subject of the sentence.

It is the thing that is somewhere:

• The stone is on the road.

In simple sentences like this, the subject is normally in the nominative.

Because the preposition á takes the dative case when it describes location.

Here the sentence means that the stone is located on the road, so Icelandic uses dative:

• dictionary form: vegur
• dative singular: vegi
• dative singular definite: veginum

So:

• á veginum = on the road

Using vegur or vegurinn here would be the wrong case.

No. Á can take different cases depending on meaning.

A very common pattern is:

• dative for location: where something is
• accusative for motion toward something: where something goes

So:

• Steinninn er á veginum = the stone is on the road, location, so dative
• Steinninn fer á veginn = the stone goes onto the road, motion, so accusative

This location-versus-motion contrast is very important in Icelandic.

Because Icelandic nouns change form for case, and sometimes the nominative form looks quite different from the other cases.

For vegur, the singular forms are:

• nominative: vegur
• accusative: veg
• dative: vegi
• genitive: vegar

With the definite article:

• nominative: vegurinn
• accusative: veginn
• dative: veginum

So veginum may look far from vegur, but it is a normal inflected form of the same noun.

Not exactly.

Icelandic does not simply take the dictionary form and stick one unchanging article onto it. Instead, both the noun form and the definite ending change according to case, number, and gender.

Here the process is roughly:

• start with vegur
• put it in dative singular: vegi
• add the matching definite ending: veginum

So veginum is a fully inflected form, not just vegur + the.

Er is the present-tense form of að vera, meaning to be.

Here it means is:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is

Since steinninn is singular, er is the correct form.

Both steinn and vegur are masculine nouns.

Yes, that matters a lot in Icelandic, because gender affects:

• noun endings
• definite article endings
• adjective endings
• some pronouns

So part of the reason you get forms like steinninn and veginum is that these are masculine nouns following masculine patterns.

Yes, but Steinninn er á veginum is the most neutral, straightforward order.

You can also move parts of the sentence for emphasis, for example:

• Á veginum er steinninn

That gives more focus to on the road. Icelandic word order is more flexible than English, but the neutral pattern here is very natural and common.