Learn Hardcore Icelandic: Starfsmennirnir undirrituðu samninginn í dag.

The subject is Starfsmennirnir (“the employees”). It’s nominative plural and the verb undirrituðu is 3rd person plural, agreeing with it. The direct object is samninginn (accusative, definite), and í dag is a time adverbial.

What does the ending on starfsmennirnir mean?

It’s the definite article in the nominative plural. Indefinite plural is starfsmenn (“employees”); add the definite ending and you get starfsmennirnir (“the employees”). In Icelandic, the definite article is usually a suffix.

How do we get from starfsmaður to starfsmennirnir?

Base noun: starfsmaður (“employee”; literally “work-person”).
Irregular plural: starfsmenn (“employees”).
Definite nominative plural: starfsmennirnir (“the employees”). Note the irregular change maður → menn.

What tense and person is undirrituðu, and what’s the infinitive?

undirrituðu is 3rd person plural past (preterite) of að undirrita (“to sign”). Mini-paradigm:

Present: (þeir) undirrita
Past: (ég) undirritaði, (þeir) undirrituðu
Past participle: undirritað

Why is samninginn in that form?

samninginn is accusative singular definite of samningur (“contract”). As the direct object of undirrita, it takes the accusative, and the definite article -inn is suffixed: samningur → samning → samninginn. The double nn comes from adding the article.

Could I say it without definites?

Yes: Starfsmenn undirrituðu samning í dag. (“Employees signed a contract today.”)

What case does í take in í dag, and why not daginn?

In time expressions, í takes the accusative to mark a point in time, hence í dag (“today”). Í daginn literally means “in the day” and isn’t used for “today.”

Can I change the word order?

Yes. Icelandic is verb-second (V2). You can front another element and keep the finite verb second:

Í dag undirrituðu starfsmennirnir samninginn.
Samninginn undirrituðu starfsmennirnir í dag. The meaning is the same; the focus shifts.

Is there a difference between undirrita and skrifa undir?

Both mean “to sign.”

undirrita is more formal/bookish.
skrifa undir is more colloquial (“write under”). Your sentence could also be: Starfsmennirnir skrifuðu undir samninginn í dag.

How would I say “The contract was signed today”?

Use the passive: Samningurinn var undirritaður í dag. (Participial form undirritaður agrees with masculine singular.)

Any quick pronunciation tips for this sentence?

ð in undirrituðu is like English “th” in “this.”
í in í dag is a long “ee.”
Primary stress is on the first syllable of each word: STARFSmennirnir, UNDIRrituðu, SAMninginn.

Where does negation go?

After the finite verb: Starfsmennirnir undirrituðu ekki samninginn í dag.