Kötturinn er feitur.

Because kötturinn means the cat, while köttur means a cat or just cat in the dictionary form.

Icelandic usually puts the definite article onto the end of the noun as a suffix instead of using a separate word like English the.

• köttur = cat
• kötturinn = the cat

So in this sentence, the speaker is talking about a specific cat.

In kötturinn, the ending -inn is the definite article, meaning the.

More specifically, here it matches a masculine singular nominative noun. Icelandic articles change form depending on gender, number, and case.

So kötturinn can be understood as:

• köttur = cat
• -inn = the

Together: the cat

The dictionary form is köttur.

When you look nouns up in a dictionary, you usually find them in the indefinite nominative singular form. So even though the sentence has kötturinn, the headword you would search for is köttur.

Because Icelandic adjectives have to agree with the noun they describe.

Here, kötturinn is:

• masculine
• singular
• nominative

So the adjective also has to be masculine singular nominative, which is feitur.

This is a very important feature of Icelandic grammar. Adjectives change form depending on the noun.

For example, with different genders, the adjective would change:

• feitur = masculine
• feit = neuter
• feitur / feit / feita etc. depending on form and context

So feitur is the correct matching form for kötturinn here.

You usually learn the gender of an Icelandic noun along with the word itself.

The noun köttur is a masculine noun. One clue is that many masculine nouns in the nominative singular end in -ur, although that is only a pattern, not a perfect rule.

Since köttur is masculine, related words like adjectives and articles must match that gender:

• köttur = a cat
• kötturinn = the cat
• feitur = fat, masculine form

Er is the present tense form of the verb að vera, which means to be.

So:

• að vera = to be
• er = is / am / are in certain forms

In this sentence, er means is.

A few present-tense forms of að vera are:

• ég er = I am
• þú ert = you are
• hann/hún/það er = he/she/it is
• við erum = we are
• þið eruð = you are
• þeir/þær/þau eru = they are

So kötturinn er feitur follows the normal pattern: subject + verb + adjective

It is in the nominative case.

That is because kötturinn is the subject of the sentence: it is the thing being described as fat.

In simple sentences with to be, the subject is usually nominative. So here:

• kötturinn = nominative subject
• er = is
• feitur = adjective agreeing with the subject

Yes. Kötturinn er feitur has the normal, basic Icelandic word order for a simple statement:

• subject
  • verb
    • complement

So:

• Kötturinn = subject
• er = verb
• feitur = adjective describing the subject

This is very similar to English The cat is fat.

You sometimes can, but it would sound marked or literary, not like the most basic neutral sentence.

For example, something like Feitur er kötturinn is possible in special styles or for emphasis, but it is not the standard beginner sentence pattern.

For ordinary neutral speech, Kötturinn er feitur is the form to use.

A rough learner-friendly guide would be:

• Kötturinn ≈ KHUHT-ur-rinn
• er ≈ ehrr
• feitur ≈ FAY-tur

A few helpful notes:

• ö is not the English o. It is closer to the vowel in some pronunciations of bird or fur, though not exactly the same.
• tt is pronounced as a clear double consonant.
• The r in Icelandic is usually a trilled or tapped r, not the typical English r.
• ei in feitur sounds like ay in say, though again not exactly identical.

If you want to sound natural, listening to native audio is especially important for Icelandic.

That happens because the definite article is added to the noun, and the result is not always just a simple one-piece ending.

In practice, you should learn kötturinn as the correct definite nominative singular form of köttur.

So even though it may look like:

• köttur
• plus something like -inn

the combined form becomes:

• kötturinn

This kind of change is normal in Icelandic inflection, and it is best to get used to learning full forms, not just trying to glue endings on mechanically every time.

It can simply be descriptive, but like English fat, it can also sound blunt depending on context.

With animals, it may sound fairly straightforward. With people, it can easily sound rude or insensitive unless used very carefully.

So grammatically feitur is correct, but in real life you should pay attention to tone and context.

Usually, no. Bare singular nouns in Icelandic do not work exactly like English a/an in every context.

While köttur is the indefinite form of cat, a sentence meant to express A cat is fat in a natural context would depend on what exactly you mean.

For beginners, the most important thing is this contrast:

• köttur = cat / a cat
• kötturinn = the cat

But Icelandic does not always use the indefinite form in exactly the same way English uses a.

Yes. Unlike German, Icelandic does not capitalize all nouns.

So:

• kötturinn is lowercase
• feitur is lowercase

You only capitalize the first word of the sentence and proper names, so:

• Kötturinn er feitur.

That capital K is there only because it starts the sentence.