Learn Hardcore Icelandic: Maturinn er vondur.

You are not replacing -ur with -in; you are adding -inn to the existing word. That is why you see -urinn at the end: it is just -ur (part of the noun) + -inn (the article).

What gender is maturinn, and why does that matter?

Matur is a masculine noun. This matters because adjectives must agree with the noun in:

gender (masculine / feminine / neuter)
number (singular / plural)
case (nominative, accusative, dative, genitive)

In Maturinn er vondur, the adjective vondur is in masculine singular nominative to match maturinn (which is also masculine singular nominative).

If the noun were feminine or neuter, the adjective would change form.

Why does the adjective end in -ur: vondur?

The adjective vondur is in its strong masculine nominative singular form, which typically ends in -ur.

For this adjective, the main nominative singular forms are:

masculine: vondur
feminine: vond
neuter: vont

Because maturinn is masculine, the correct agreeing form is vondur:

Maturinn er vondur. – The food is bad.
Súpan er vond. – The soup is bad. (feminine)
Brauðið er vont. – The bread is bad. (neuter)

Could I say Maturinn er vont instead?

No. Vont is the neuter form of the adjective.
Since maturinn is masculine, the adjective must also be masculine:

❌ Maturinn er vont. – ungrammatical
✅ Maturinn er vondur.

You would use vont with a neuter noun, e.g.:

Brauðið er vont. – The bread is bad. (bread is neuter)

If the noun is definite (maturinn), shouldn’t the adjective also be “definite”, like a weak form (vondi)?

In Icelandic there are two main kinds of adjective endings:

strong endings (like vondur)
weak endings (like vondi)

Weak endings are mainly used for attributive adjectives (adjectives placed directly before a definite noun):

góður matur – good food (indefinite, strong)
góði maturinn – the good food (definite noun, weak adjective)

However, in predicative position (after vera = to be), adjectives usually take the strong endings, even when the noun is definite:

Maturinn er vondur. – The food is bad. (strong)
Maðurinn er góður. – The man is good. (strong)

So Maturinn er vondur is correct; Maturinn er vondi would be wrong here.

Which case is maturinn in, and why?

Maturinn is in the nominative case.

In a simple sentence with vera (to be), the subject appears in the nominative:

Maturinn er vondur. – The food is bad.
Strákurinn er þreyttur. – The boy is tired.

The adjective vondur is also in nominative because it agrees with the subject.

What does er mean, and how does it change in other tenses?

Er is the present tense, 3rd person singular of the verb vera (to be):

ég er – I am
þú ert – you are (singular)
hann / hún / það er – he / she / it is

Other common forms:

var – was (3rd person singular past)
- Maturinn var vondur. – The food was bad.
verður – will be (3rd person singular future)
- Maturinn verður vondur. – The food will be bad.

In your sentence, er simply corresponds to English is.

Is vondur only “bad-tasting”, or can it also mean “evil”?

Vondur is quite flexible. It can mean:

bad, unpleasant, poor (quality)
- Maturinn er vondur. – The food is bad (e.g. tastes bad).
nasty, unpleasant (about situations, conditions, feelings)
- Veðrið er vont. – The weather is bad.
- Mér líður illa. – I feel bad. (here using illa, the adverb)
evil, wicked (morally) – often about people or characters
- Hann er vondur maður. – He is an evil/bad man.
- Vonda nornin – the evil witch.

Another common adjective for bad in a more neutral, less “evil” sense is slæmur:

Maturinn er slæmur. – The food is bad / of poor quality.

In everyday speech, vondur is extremely common and context tells you whether it’s “bad” or “evil.”

How would I say this as a yes–no question: “Is the food bad?”

You usually keep the same word order and just use question intonation, or you add a question marker like er at the start in more formal styles.

Most natural spoken version:

Maturinn er vondur? – rising intonation = Is the food bad?

More clearly marked question (still correct):

Er maturinn vondur? – Is the food bad?

So the two main options:

Maturinn er vondur?
Er maturinn vondur?

How do I say “The food is not bad”?

You use ekki (not) after the verb er:

Maturinn er ekki vondur. – The food is not bad.

Word order: subject – verb – ekki – adjective is the normal pattern here.

What is the typical word order? Could I say Vondur er maturinn?

The neutral word order in Icelandic, especially in simple statements, is:

Subject – Verb – Complement

So:

Maturinn er vondur. – Subject (maturinn) + verb (er) + adjective (vondur).

You can say Vondur er maturinn, but it is marked and sounds more like:

“It is bad that food (in particular)”
or a kind of emphatic, poetic, or contrastive structure, stressing the adjective vondur.

For normal, everyday speech, Maturinn er vondur is the natural choice.

How is Maturinn er vondur pronounced?

Approximate pronunciation (IPA and notes):

maturinn – [ˈmaː.tʏ.rɪn]
- ma- like maa in mart but longer [aː]
- -t- is a clear [t]
- -ur- [ʏr] (like a short u in put
  - rolled/typed r)
- stress on the first syllable: MA-tur-inn
er – [ɛr]
- like air but shorter, with an r at the end.
vondur – [ˈvɔn.tʏr]
- vo- like vo in volleyball, but with a more open [ɔ]
- -nd- is pronounced [nt] (see next question)
- -ur as [ʏr] again
- stress on the first syllable: VON-dur

Altogether: [ˈmaː.tʏ.rɪn ɛr ˈvɔn.tʏr].

Why is nd in vondur pronounced like nt?

In Icelandic, the consonant cluster nd is usually pronounced [nt]. This is a regular sound change:

vondur – spelled with d, but pronounced [ˈvɔn.tʏr]
landi – pronounced [ˈlan.tɪ]
hundur – pronounced [ˈhʏn.tʏr]

So it is just a spelling–pronunciation mismatch that learners have to memorize: nd → [nt] in most positions.