Fuglinn flýr hundinn.

In Icelandic, the definite article is usually attached to the end of the noun instead of being a separate word.

• fuglinn = fugl
  • -inn = the bird
• hundinn = hund
  • -inn = the dog (here in the accusative form)

So Icelandic often says in one word what English says in two.

Because the two nouns are in different cases.

• fuglinn is the subject, so it is in the nominative
• hundinn is the object, so it is in the accusative

The noun hundur changes form by case:

• nominative definite: hundurinn = the dog as subject
• accusative definite: hundinn = the dog as object

So hundinn shows that the dog is the one being fled from, not the one doing the fleeing.

Mostly from the case forms.

• fuglinn is nominative singular definite
• hundinn is accusative singular definite

Icelandic uses case endings much more than English does. In this sentence, the endings tell you who is doing the action and who receives it.

Word order also helps, since subject–verb–object is the most neutral order here, but the case endings are the strongest clue.

The dictionary form is að flýja, meaning to flee or to run away from.

flýr is the 3rd person singular present tense form, used with he/she/it and singular nouns like fuglinn.

So:

• að flýja = to flee
• ég flý = I flee
• þú flýrð = you flee
• hann/hún/það flýr = he/she/it flees

Because Icelandic flýja can take a direct object.

So Icelandic can say:

• flýja hundinn = flee the dog / flee from the dog

In natural English, we usually say flee from the dog, but Icelandic does not need a preposition here.

Yes, that is possible, but it is built differently.

• Fuglinn flýr hundinn uses flýja with a direct object in the accusative.
• Fuglinn flýr frá hundinum uses the preposition frá = from, which requires the dative case, so hundinum is dative.

The version without frá is very normal and compact in Icelandic.

Both nouns here are masculine:

• fugl = bird, masculine
• hundur = dog, masculine

That matters because Icelandic noun endings, article endings, and adjective endings depend on gender, number, and case.

That is a normal noun inflection pattern.

The basic nominative singular indefinite form is:

• hundur = a dog

But when the noun changes case, the ending -ur often disappears before other endings are added:

• accusative indefinite: hund
• accusative definite: hundinn

So this is not a different word, just a different grammatical form of the same noun.

You could, because Icelandic word order is more flexible than English word order. The case endings still show the roles:

• fuglinn is still the subject
• hundinn is still the object

But Fuglinn flýr hundinn is the most neutral way to say it. Putting hundinn first gives it more emphasis or a more marked style.

The Icelandic present tense can cover several meanings depending on context.

So Fuglinn flýr hundinn can mean things like:

• The bird flees the dog
• The bird is fleeing the dog

English chooses between simple present and progressive more often than Icelandic does. Icelandic present tense is broader.

Roughly speaking, ý is pronounced like a long ee sound, and the word is approximately like fleer in English, though not exactly.

A very rough guide:

• Fuglinn ≈ FOO-glin
• flýr ≈ fleer
• hundinn ≈ HUN-din

The stress in Icelandic normally falls on the first syllable of the word.

Because Icelandic can express a lot of grammar directly in the noun and verb forms.

In this short sentence, you already get all of this from the forms alone:

• who is doing the action
• who is affected by it
• that both nouns are definite
• that the subject is singular
• that the verb is present tense

That is one reason Icelandic sentences can look very compact compared with English.