Hún felur lykilinn undir mottunni þegar við leikum feluleik í garðinum.

Fela is the infinitive, meaning the basic dictionary form to hide.

Felur is the 3rd person singular present tense, used with hún (she):

• ég fel = I hide
• þú felur = you hide
• hún felur = she hides
• við felum = we hide

So Hún felur ... means she hides ...

Because lykilinn is the direct object of the verb felur, and in Icelandic that object is in the accusative case here.

The noun is:

• lykill = a key
• lykil = key, accusative singular
• lykilinn = the key, accusative singular definite

So the form shows two things at once:

1. it is accusative
2. it is definite = the key

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

So:

• lykill = a key

• lykilinn = the key

• motta = a mat

• mottunni = the mat

• garður = a garden
• garðinum = the garden

This ending changes depending on gender, number, and case, so the article is not one single fixed form.

Yes. Undir can take either accusative or dative, depending on the meaning.

A very common rule is:

• accusative = movement/direction toward a position
• dative = location in a position

In this sentence, undir mottunni uses the dative and presents under the mat as the place where the key is hidden.

So:

• undir mottunni = under the mat, as a location
• undir mottuna = under the mat, with more emphasis on putting/moving it there

In real language, with verbs like hide, the line can feel a bit blurry, because hiding often involves both movement and resulting location. But the sentence you were given chooses the locational reading.

The basic form is motta = mat or rug.

In the sentence, mottunni is dative singular definite.

You can think of it like this:

• motta = a mat
• mottu = mat, dative/accusative singular (indefinite form of the noun stem in those cases)
• mottunni = the mat, dative singular

The ending changes because the noun is:

• feminine
• singular
• definite
• after a preposition that takes the dative here

Because leika is the infinitive, while leikum is the 1st person plural present tense that matches við (we).

So:

• leika = to play
• við leikum = we play

The verb has to agree with the subject:

• ég leik = I play
• þú leikur = you play
• við leikum = we play

So after við, you need leikum, not the infinitive leika.

Feluleikur is the noun hide-and-seek. In the sentence it appears as feluleik, which is the accusative singular form.

It is a compound noun:

• felu- = from the idea of hiding
• leikur = game

So literally it is something like a hiding game.

It does not have the definite article here because Icelandic commonly uses this game name without it in the expression:

• leika feluleik = play hide-and-seek

That works much like English, where we usually say play hide-and-seek, not play the hide-and-seek.

If you were talking about one specific game already known from context, you could use the definite form feluleikinn.

Because í can also take accusative or dative, and here it means location, not movement.

So:

• í garðinum = in the garden, location
• í garðinn = into the garden, movement toward/into

Since the sentence means playing in the garden, the noun is in the dative:

• garðinum = the garden, dative singular definite

Because þegar introduces a subordinate clause, and subordinate clauses in Icelandic normally do not use the main-clause verb-second pattern in the same way.

So after þegar, the normal order is:

• þegar við leikum ... = when we play ...

not:

• þegar leikum við ...

A useful comparison:

• Hún felur lykilinn ... = main clause
• þegar við leikum feluleik ... = subordinate clause

If you put the þegar clause first, then the following main clause would show the normal main-clause pattern:

• Þegar við leikum feluleik í garðinum, felur hún lykilinn undir mottunni.

Because the sentence describes something habitual or repeated: this is what happens when or whenever that situation occurs.

Icelandic often uses the simple present for this kind of meaning, just like English can:

• She hides the key under the mat when we play hide-and-seek in the garden.

So:

• felur = hides
• leikum = play

This is not necessarily about one single event happening right now. It can describe a repeated pattern.

Yes — this sentence is a very good example of how Icelandic uses case to show each noun’s role.

• Hún = subject, nominative
• lykilinn = direct object, accusative
• undir mottunni = prepositional phrase with dative
• feluleik = object of leikum, accusative
• í garðinum = prepositional phrase with dative

So an Icelandic learner can see several things at once:

1. subject form
2. object form
3. prepositions that control case
4. definite article attached to nouns

That is one reason a short sentence like this is actually very useful for grammar practice.