Hún hleypir hundinum inn.

Hleypir is from the verb hleypa, which often means to let, to allow, or to make something move/go in this kind of sentence.

That is different from hlaupa, which means to run.

So:

• hún hleypir hundinum inn = she lets the dog in
• hún hleypur inn = she runs in

This is a very important distinction:

• hlaupa = the subject runs
• hleypa = the subject lets or causes someone/something to go

Because hleypa in this meaning takes the dative case, not the accusative.

So the noun hundur changes like this in the singular definite form:

• nominative: hundurinn = the dog (as subject)
• accusative: hundinn
• dative: hundinum

In Hún hleypir hundinum inn, hundinum is the thing being allowed in, and with this verb Icelandic uses the dative.

This is something English speakers often just have to learn with the verb:

• hleypa einhverjum inn = let someone in

Here einhverjum is the dative form of einhver.

The dictionary form is hleypa.

Hleypir is the 3rd person singular present tense form, used with hann / hún / það:

• ég hleypi
• þú hleypir
• hann / hún / það hleypir

So Hún hleypir hundinum inn literally has:

• hún = she
• hleypir = lets
• hundinum = the dog
• inn = in

Inn means in / inside / inward and shows the direction of movement.

So hleypa hundinum inn means:

• let the dog in
• literally, let the dog inward/inside

Without inn, the sentence would feel less specific. Inn tells you the dog is being allowed to go from outside to inside.

This kind of directional adverb is very common in Icelandic:

• inn = in
• út = out
• upp = up
• niður = down

No, here inn is not a preposition. It is an adverb/particle of direction.

A preposition would normally be followed by a noun phrase, but here inn stands on its own.

Compare:

• Hún hleypir hundinum inn. = She lets the dog in.
• Hún fer inn í húsið. = She goes into the house.

In the second sentence, í is the preposition, and inn adds the directional sense.

This is the normal Icelandic main-clause word order:

• subject: Hún
• verb: hleypir
• object/complement: hundinum
• directional particle: inn

So it works very much like English:

• She lets the dog in

Icelandic often puts the finite verb in the second position in main clauses, but in a simple sentence like this, that gives you the familiar pattern subject + verb + rest.

That would not be the normal word order here.

The natural order is:

• Hún hleypir hundinum inn.

The directional particle inn usually comes after the noun phrase here, much like English let the dog in rather than let in the dog in a neutral sentence.

In this sentence, hún is the feminine 3rd person singular pronoun, so it means she.

The Icelandic 3rd person singular pronouns are:

• hann = he
• hún = she
• það = it

So the subject is clearly feminine singular.

You would just change the subject pronoun:

• Hann hleypir hundinum inn.

The verb form hleypir stays the same because both hann and hún take the same 3rd person singular present form.

Yes. You can expand the sentence if you want to specify where the dog is being let in.

For example:

• Hún hleypir hundinum inn í húsið. = She lets the dog into the house.

Here:

• inn gives the directional idea
• í húsið specifies the destination

That said, Hún hleypir hundinum inn already works perfectly well if the destination is understood from context, just like English She lets the dog in.

The base noun is hundur = dog.

In this sentence you need:

• singular
• definite = the dog
• dative

That gives hundinum.

A very rough breakdown is:

• hund- = noun stem
• -inum = a common dative singular definite ending for masculine nouns of this type

So learners often memorize it as:

• hundur = dog
• hundinum = to/for the dog, or the dog in dative contexts

Yes. Very naturally.

For example:

• hleypa einhverjum inn = let someone in
• hleypa einhverjum út = let someone out

The structure stays similar, and the person or thing being allowed usually stays in the dative.

So once you learn hleypa einhverjum inn, you can reuse the pattern in other expressions.

A learner-friendly approximation is:

• Hún ≈ hoon
• hleypir ≈ something like hlay-pir
• hundinum ≈ HUN-di-num
• inn ≈ in

A few notes:

• ú is like a long oo
• ey in hleypir sounds somewhat like ay
• ll / hl / hn / hr-type clusters can be tricky for English speakers; here the initial hl in hleypir may take practice
• stress in Icelandic usually falls on the first syllable

So the main stress pattern is roughly:

• HÚN HLEY-pir HUN-di-num INN

A native recording is always best, but this should help you get started.