Learn Hardcore Icelandic: Tómatar eru ferskir í dag.

Tómatar is masculine plural in the nominative case (subject).
The predicate adjective after vera (to be) also appears in the nominative.
Masculine nominative plural of ferskur is ferskir. Other nominative forms for contrast:
Masculine: ferskir (pl), ferskur (sg)
Feminine: ferskar (pl), fersk (sg)
Neuter: fersk (pl), ferskt (sg)

Why is it eru and not er?

Tómatar is plural, so you need the 3rd person plural of vera (to be). Present tense of vera:

Where is “the”? How do I say “The tomatoes are fresh today”?

Icelandic doesn’t have a separate definite article; it’s a suffix.

Indefinite/general: Tómatar eru ferskir í dag.
Definite/specific: Tómatarnir eru ferskir í dag. Use the definite only when you mean specific tomatoes known from context.

Can I put í dag first? What happens to word order?

Yes. Icelandic main clauses are verb‑second (V2):

Neutral: Tómatar eru ferskir í dag.
Fronted time phrase: Í dag eru tómatar ferskir. Yes/no question by inversion:
Eru tómatar ferskir í dag?

What case are tómatar and ferskir in?

Tómatar is nominative plural (it’s the subject).
After vera (to be), the predicate adjective also takes nominative: ferskir (not accusative or dative).

Why is it í dag (“in day”)—what case is dag here?

It’s a fixed time expression with the preposition í + accusative:

í dag = “today” (accusative singular of dagur → dag). Note: í can take different cases in other contexts (e.g., location vs. motion), but for “today,” it’s a set phrase in the accusative.

How do I say “There are fresh tomatoes today”?

Use an existential clause with dummy Það:

Það eru ferskir tómatar í dag. This states availability/existence, not a property of a known set.

How would the sentence change with feminine or neuter nouns?

Adjectives still agree:

How do I negate it?

Place ekki after the verb:

Tómatar eru ekki ferskir í dag. (Tomatoes are not fresh today.) Stronger: Engir tómatar eru ferskir í dag. (No tomatoes are fresh today.)

What are the key forms of tómatur I’ll actually use?

Singular: nominative tómatur, accusative tómata, dative tómati, genitive tómats
Plural: nominative tómatar, accusative tómata, dative tómötum (note ö), genitive tómata Examples:
Ég kaupi tómata. (I buy tomatoes – acc.)
Ég elda með tómötum. (I cook with tomatoes – dat.)

How does ferskur decline and how do I compare it?

Basic nominative forms:

m sg ferskur, f sg fersk, n sg ferskt
m pl ferskir, f pl ferskar, n pl fersk Comparatives:
Comparative: ferskari (e.g., Tómatar eru ferskari í dag en í gær.)
Superlative (attributive): ferskastur/ferskust/ferskast
Superlative (predicative): ferskastur etc. (e.g., Þessir tómatar eru ferskastir.)

Pronunciation tips for the whole sentence?

Tómatar: stress the first syllable (TOH-), ó like English “oh”, final r is trilled.
eru: roughly “EH-ru” (short vowels; the r is tapped).
ferskir: stress on fer-; sk before i/e is like “sh” (so “FER-shkir”).
í dag: í like “ee”; g in dag sounds like a soft “gh” (≈ “dahgh”). Icelandic always stresses the first syllable of words.

Is the sentence generic or specific without the article?

Without the definite article it can be generic or context‑dependent:

Generic: Tómatar eru ferskir í dag. (As a general store report, “Tomatoes are fresh today.”)
Specific known set needs the definite: Tómatarnir eru ferskir í dag.

Can I use words other than ferskur for “fresh”?

ferskur: fresh (food, air, ideas), the default here.
nýr: new (not “fresh” for produce unless you mean newly arrived: nýir tómatar = newly arrived tomatoes).
frískur: healthy (of people), not used for food. Don’t say tómatar eru frískir.

Why not ferskum here?

ferskum is the dative plural form, used when the noun phrase is in the dative:

Example: með ferskum tómötum (with fresh tomatoes). In our sentence the adjective is nominative plural, so ferskir is correct.