Learn Hardcore Icelandic: Kaffið er sterkt, en sykurinn vantar. - The coffee is strong, but the sugar is missing.

Those are the definite article suffixes in Icelandic. Instead of putting “the” before a noun, you tack it on at the end. Kaffi (coffee) + -ið = kaffið (the coffee), and sykur (sugar) + -inn = sykurinn (the sugar).

Why does the adjective sterkt end with -t instead of -ur?

Adjectives agree in gender, number and case with the noun they describe. Kaffi is a neuter noun in the nominative singular, so the base adjective sterkur (strong) takes the neuter ending -t, giving sterkt.

What does en mean and why is there a comma before it?

en is a coordinating conjunction meaning but. When you join two independent clauses in Icelandic with en, you normally separate them with a comma, just as you would with but in English.

What does the verb vantar mean here, and what case is sykurinn in after it?

vanta means to be missing or to lack. In “sykurinn vantar” the sugar is the thing that’s missing, so it stays in the nominative as the grammatical subject. This verb is intransitive in this use—you say literally “the sugar is missing.”

How would I use vantar with a pronoun to say “I need sugar”?

When you want to specify who lacks something, you add a dative (actually accusative in modern usage) pronoun before or after the verb. The most common pattern is:
• Mig vantar sykur. – “I need sugar” (literally, “Me is missing sugar.”)
• For “we,” you say Okkur vantar sykur. (“Us is missing sugar,” i.e. “we need sugar.”)

Why is sykurinn definite in this sentence, and when would I leave off the article?

Here you’re talking about the sugar that ought to be there (for example, on the table), so it’s definite. If you talk about sugar in general or any sugar, you drop the article:
• Kaffi er sterkt, en sykur vantar. – “Coffee is strong, but sugar is missing/needed.”
That sounds like you want some sugar in general, not a particular batch.

How would I express “there is no sugar” or “we need sugar” more generally in Icelandic?

You have two common strategies:
1) Use vantar impersonally:
• Okkur vantar sykur. – “We need sugar.”
2) State absence with enginn:
• Það er enginn sykur. – “There is no sugar.”
Either one works depending on whether you focus on the need (vantar) or simply the lack (enginn).

How is “Kaffið er sterkt, en sykurinn vantar” pronounced? Where is the word stress?

Icelandic words almost always stress the first syllable. Rough phonetic guide:
• KAFF-ith er STERKT, en SYK-ur-in VAN-tar
– KAF (like “cuff”) + (ið) pronounced like the “ith” in “earth”
– STERKT = “stairkt” (k is hard)
– SYK = “sick” with a y ~ German ü, then –ur-in
– VAN = “van,” tar = like “tar”
So stress on KAF, STERKT, SYK, and VAN.