Feiti kötturinn sefur.

Because feiti is the weak masculine nominative singular form of the adjective feitur (fat).

In Icelandic, adjectives change form to match the noun they describe. They agree with the noun in:

• gender
• number
• case
• and often definiteness

Here, kötturinn means the cat, so the noun is definite. With a definite noun, Icelandic normally uses the weak form of the adjective:

• feitur köttur = a fat cat
• feiti kötturinn = the fat cat

So feiti is used because the noun is masculine, singular, nominative, and definite.

In Icelandic, the definite article is usually attached to the end of the noun, rather than written as a separate word the way English does it.

So:

• köttur = cat
• kötturinn = the cat

That means Icelandic often says the equivalent of fat-the-cat in structure, even though the natural English translation is the fat cat.

The exact definite form depends on the noun’s declension pattern, so it is best to learn kötturinn as the definite nominative singular form of köttur.

Both feiti and kötturinn are:

• masculine
• singular
• nominative

Why nominative? Because the fat cat is the subject of the sentence, and subjects are typically in the nominative case.

Why masculine? Because köttur is a masculine noun.

The adjective has to match the noun, so feiti is also masculine singular nominative.

Because that is the normal position for descriptive adjectives in Icelandic, just as in English.

So:

• feiti kötturinn = the fat cat

This is the standard order: adjective + noun.

What is different from English is not the adjective position, but the article. English puts the before the adjective, while Icelandic usually puts the definite article on the noun itself.

Sefur is the 3rd person singular present tense form of the verb sofa (to sleep).

So:

• sofa = to sleep
• sefur = sleeps / is sleeping

It is 3rd person singular because the subject, feiti kötturinn, is singular: the fat cat.

Unlike adjectives, Icelandic verbs do not change for gender here. The verb just matches the subject in person and number.

It can mean either, depending on context.

So Feiti kötturinn sefur can mean:

• The fat cat sleeps
  or
• The fat cat is sleeping

Icelandic often uses the simple present where English might choose either simple present or present progressive. Context tells you which meaning is intended.

Yes, but this sentence uses the most basic and natural order for a simple statement:

• Feiti kötturinn sefur.

That is:

• subject
  • verb

Icelandic does allow other elements to come first, but then the verb still usually stays in the second position in the clause. For example, if you began with a time word, the structure would shift accordingly.

So for a beginner, this sentence is a good model of the normal straightforward pattern.

You would remove the definiteness and use the strong adjective form instead:

• Feitur köttur sefur. = A fat cat sleeps.

Compare:

• feitur köttur = a fat cat
• feiti kötturinn = the fat cat

This is a very important Icelandic pattern:

• indefinite noun → usually strong adjective
• definite noun → usually weak adjective

So this sentence is useful because it shows both the definite noun and the matching weak adjective at the same time.