Feluleikur er uppáhalds leikurinn hennar.

Feluleikur is a compound noun, which is very common in Icelandic.

It is made of:

• fela = to hide
• felu- = a noun stem related to hiding
• leikur = game, play

So feluleikur is literally something like hiding-game, i.e. hide-and-seek.

Compound words are extremely common in Icelandic, often more common than separate-word expressions would be in English.

Leikurinn means the game.

Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the.

So:

• leikur = a game / game
• leikurinn = the game

In this sentence, uppáhalds leikurinn hennar means her favorite game, so the definite form is needed.

Hennar means her.

It is the genitive form used to show possession, so:

• leikurinn hennar = her game
• more naturally in English here: her favorite game

Icelandic often uses this structure where English would also use a possessive determiner.

Compare:

• bíllinn hennar = her car
• húsið hennar = her house

The given sentence is a normal and natural way to say it.

Its structure is:

• Feluleikur = subject
• er = is
• uppáhalds leikurinn hennar = predicate noun phrase

So literally:

• Hide-and-seek is her favorite game.

Icelandic often uses X er Y just like English X is Y.

You could also express the idea in other ways, but this sentence is straightforward and idiomatic.

Because this is a sentence with the verb vera (to be), and both sides of an X is Y sentence are often in the nominative in Icelandic.

So:

• Feluleikur = nominative
• leikurinn = nominative

This is similar to what grammarians call a predicate nominative.

The basic idea is that the sentence is identifying one thing as another, not showing one thing acting on another.

Uppáhalds means favorite, but it does not behave like a regular adjective in all ways.

Historically and structurally, it comes from uppáhald (favorite thing, something especially liked), and in modern Icelandic it is commonly used before a noun in a fairly fixed way:

• uppáhalds bókin mín = my favorite book
• uppáhalds liturinn hennar = her favorite color
• uppáhalds leikurinn hennar = her favorite game

A learner can usually treat uppáhalds as an indeclinable modifier meaning favorite.

You may see variation here.

In modern usage, uppáhalds is often written separately before the noun:

• uppáhalds leikurinn hennar

But compounds in Icelandic are very common, and in some contexts learners may also encounter forms written more tightly together, especially in less formal usage or in learner expectations based on how compounds usually work.

For practical learning, it is safest to remember the very common pattern:

• uppáhalds + noun

So here:

• uppáhalds leikurinn hennar

This is a very common learner question.

Sinn / sín / sitt is a reflexive possessive, used when the possessor refers back to the subject in certain contexts, especially inside subordinate structures or where reflexive possession is needed.

But in a simple sentence like this, hennar is the normal way to say her:

• Feluleikur er uppáhalds leikurinn hennar.

Using sinn here would not be the standard choice.

A simple learner rule is:

• use hans / hennar / þeirra for ordinary his / her / their
• learn sinn as a special reflexive possessive used in more specific grammatical situations

You could emphasize hennar in speech, but the basic wording usually stays the same:

• Feluleikur er uppáhalds leikurinn hennar.

If you want stronger contrast, Icelandic can also use word order or stress depending on context.

For example, if the hidden contrast is hers, not someone else’s, the spoken emphasis would fall on hennar.

The written sentence itself does not have to change.

A rough pronunciation guide for an English speaker:

• uppáhalds ≈ UP-pow-halts
• leikurinn ≈ LAY-ku-rin
• feluleikur ≈ FEH-lu-lay-kur

A few helpful notes:

• ei in leikur sounds like ay in say
• ll in uppáhalds here is not pronounced like English ll in a fully simple way; Icelandic consonants often shift a bit depending on position
• the stress in Icelandic is usually on the first syllable

So the main stress is on:

• FEluleikur
• UPpáhalds
• LEIkurinn

It is understandable, but it is not the most natural equivalent of the given sentence.

The original sentence:

• Feluleikur er uppáhalds leikurinn hennar

is a very natural way to say Hide-and-seek is her favorite game.

Your alternative changes the structure and may sound less idiomatic or at least different in emphasis. Icelandic often prefers the definite noun phrase here:

• uppáhalds leikurinn hennar = her favorite game

So for a learner, the original sentence is the best model to remember.

They are related but do different jobs.

• feluleikur names the category or game: hide-and-seek
• leikurinn hennar means her game / the game of hers, and with uppáhalds it becomes her favorite game

So the sentence is identifying one specific game type as the one that is her favorite.

You can think of it like:

• [Hide-and-seek] = [her favorite game]

That is why one noun phrase is indefinite in form (feluleikur) and the other is definite (leikurinn hennar).