Learn Hardcore Icelandic: Lestin sem fer vestur er sein.

The -in is the definite article attached to the noun. lest means “a train,” while lestin means “the train.” In Icelandic, definiteness is shown by suffixing the article to the noun rather than using a separate word like the.

Why is sem used after lestin? Could we drop it, as English often drops “that”?

sem is the invariable relative pronoun in Icelandic (covering English “that,” “which,” “who”). You cannot drop it; Icelandic requires a relative pronoun before a relative clause.

Why is there no comma before sem?

Icelandic does not set off restrictive relative clauses with commas. The clause sem fer vestur (“that goes west”) is integrated directly into the noun phrase.

Why is vestur not preceded by til, as in til vesturs?

With motion verbs like fara (“to go”), you can either say fara vestur (use vestur as an indeclinable adverb) or fara til vesturs (a prepositional phrase using the genitive). Both are correct; the shorter form is very common in speech and writing.

Is vestur a noun or an adverb here?

Here vestur functions as an adverb of direction (“west”). It is indeclinable in this use and simply tells you which way the train is going.

Why is the adjective sein not seinn, seint, or seinna?

Adjectives in a predicate with a definite subject normally appear in their strong, uninflected form. Moreover, lestin is feminine, and the positive form of “late” for feminine is sein. seinn is masculine, seint is neuter, and seinna is the adverbial/comparative form meaning “later.”

Why do we have two instances of the verb er (once as part of sem fer and once before sein) and not just one?

Actually, fer in sem fer vestur is the present‐tense of fara (“it goes”). The second er is the main clause’s verb “is” linking the subject to the predicate adjective sein. You need both because one belongs inside the relative clause, the other is the main verb of the sentence.

Could we express “is going” in the main clause more explicitly, like “the train that is going west is running late”?

Yes. If you want to emphasize the ongoing action, you can say:
Lestin sem er að fara vestur er sein.
Here er að fara is the present progressive construction (“is going”), but it’s optional; sem fer vestur already conveys “that goes/is going west.”

What word order rule is at play here?

Icelandic is a verb‐second (V2) language in main clauses. The first constituent is Lestin, the second is the finite verb er, and everything else follows. The entire relative clause sem fer vestur counts as part of the first constituent, so er remains in second position.

Could we say Lest sem fer vestur er sein without the definite article?

You could if you mean “A train that goes west is late,” but that feels odd unless you’re talking about trains in general. Most of the time you refer to a specific train, so you use the definite lestin.