Learn Hardcore Icelandic: Ég kaupi ekkert nema tómata.

Because the noun is a direct object and needs the accusative plural. With the verb kaupa (“to buy”), objects go in the accusative. The word after nema (“except”) keeps the case it would have as the object:

Nom. pl.: tómatar (“tomatoes” as the subject)
Acc. pl.: tómata (“tomatoes” as the object)
Acc. pl. definite: tómatana (“the tomatoes” as the object)

If you remove the exception phrase, you get the normal object: Ég kaupi tómata.

What case is ekkert here, and why does it end in -t?

Functionally it’s accusative (it’s the object of kaupi), but in neuter singular the nominative and accusative look the same. Ekkert is the neuter singular form of a negative pronoun meaning “nothing.” The -t ending marks neuter singular in nom/acc, just like in adjectives: stórt, gott, and the numeral eitt.

What exactly does nema do here?

With a noun phrase, nema means “except/but.” It does not assign a case; the following noun appears in whatever case the context requires: nema tómata (accusative because it’s the object of kaupi).
With a full clause, nema means “unless”: for example, Ég kem ekki, nema þú komir (“I won’t come unless you come”). You may also see nema að before clauses.

Can I say ekki nema tómata instead of ekkert nema tómata? Is there a difference?

Yes. Ég kaupi ekki nema tómata is very common and means “I buy only tomatoes / nothing but tomatoes.”
Both ekkert nema X and ekki nema X convey the same idea. Nuance:

ekkert nema X = literally “nothing except X,” slightly more emphatic or formal.
ekki nema X = common, natural in everyday speech (“only X”). You can also use bara/aðeins/einungis: Ég kaupi bara tómata (“I only buy tomatoes”).

Where would ekki go if I use the ‘ekki neitt’ version?

Right after the finite verb (V2 word order):
Ég kaupi ekki neitt nema tómata.
Avoid placing ekki before the verb here.

Is “ekki ekkert” allowed?

Generally no. It’s felt as an odd double negation. Use either:

Ég kaupi ekkert (“I buy nothing”), or
Ég kaupi ekki neitt (“I don’t buy anything”). Both are standard; don’t combine ekki with ekkert.

How would I say “I buy no tomatoes” (without the exception)?

Use the “no + noun” pattern with enginn:
Ég kaupi enga tómata.
Here enga is the accusative plural of enginn agreeing with tómata.

Why is there no word for “a” or “some” before tómata?

Icelandic has no indefinite article. A bare noun is indefinite by default. If you want “the tomatoes,” you use the definite suffix:

Indefinite: tómata (“tomatoes”)
Definite: tómatana (“the tomatoes”) So: Ég kaupi ekkert nema tómatana = “I buy nothing except the tomatoes.”

What form is kaupi, and how does kaupa conjugate in the present?

Kaupi is 1st person singular present indicative of kaupa (“to buy”). Present tense:

ég kaupi
þú kaupir
hann/hún/það kaupir
við kaupum
þið kaupið
þeir/þær/þau kaupa Past: ég keypti; past participle: keypt.

How do I say “I’m buying …” (progressive)?

Icelandic usually uses simple present for both “I buy” and “I’m buying.” If you need to stress an ongoing action, use vera að + infinitive:

Ég er að kaupa tómata.

Can I drop Ég and just say “Kaupi …”?

Normally no. Icelandic is not a pro-drop language; the subject pronoun is required in standard sentences. You might omit it in headlines, notes, or instructions, but not in ordinary conversation.

Can I front something for emphasis? How does word order react?

Yes—Icelandic is a V2 language. Whatever you front, the finite verb stays second:

Neutral: Ég kaupi ekkert nema tómata.
Emphatic fronting: Ekkert kaupi ég nema tómata. You can also front the exception phrase for a marked, stylistic effect: Nema tómata kaupi ég ekkert.

Does nema force any special punctuation?

Not with a simple noun phrase: Ég kaupi ekkert nema tómata (no comma).
With a full clause, you’ll usually see a comma in writing: Ég kem ekki, nema þú komir.

Pronunciation tips for the whole sentence?

Very rough guide (English-like approximation):

Ég ≈ “yeh/yegh” (the g is soft, often barely audible)
kaupi ≈ “KOY-pi” (au like fronted “oy”)
ekkert ≈ “EH-kert” (kk like a hard k; rt as in “car-t” with a tapped/voiceless r)
nema ≈ “NEH-ma”
tómata ≈ “TOH-ma-ta” (ó like “oh”; stress on first syllable) Icelandic stress is almost always on the first syllable.

Why is tómata plural here? Could I make it singular?

Plural fits the idea of “tomatoes” in general. If you literally mean one tomato, use the singular accusative:

Ég kaupi ekkert nema (einn) tómat. Here tómat is singular accusative of tómatur, and einn (“one”) agrees in case and gender.

What’s the difference between ekkert, neitt, and enginn?

ekkert = “nothing” (neuter pronoun by itself): Ég geri ekkert (“I do nothing”).
neitt = “anything/something (at all)” used with negation or questions: Ég geri ekki neitt (“I don’t do anything”).
enginn (m.), engin (f.), ekkert (n.) = “no/none” used with nouns, and “no one/nobody” for people:
- “no tomatoes”: enga tómata (acc. pl.)
- “no one”: enginn (as a person).

Does nema affect case, or does the following word just take whatever case the context needs?

Nema doesn’t assign a case. The following noun/pronoun appears in the case required by the verb or preposition:

Direct object (acc.): Ég kaupi ekkert nema tómata.
With a preposition (acc.): Ég tala við engan nema þig.
With a preposition (dat.): Ég fæ ekkert frá neinum nema frá honum.
With a preposition (gen.): Ég hef ekkert til nema peninga.