Hann er í fangelsinu.

Er is the 3rd person singular present form of the verb að vera (to be).

So:

• ég er = I am
• þú ert = you are
• hann er = he is

Icelandic verbs change form depending on the subject, so er is simply the correct form for hann.

Hann means he.

It is the masculine singular nominative pronoun, used when a male person is the subject of the sentence.

Compare:

• hann = he
• hún = she
• það = it

In this sentence, hann is the subject, so the nominative form hann is used.

Í usually means in.

It is a very common preposition and is used for being inside something or in a place.

Examples:

• í húsi = in a house
• í skólanum = in the school
• í fangelsinu = in the prison / in prison

A very important thing for learners is that í can take different cases depending on meaning:

• accusative for motion into something
• dative for location in something

Here it describes location, not movement, so it takes the dative.

The ending -inu shows that the noun is:

• singular
• definite (the prison)
• in the dative case

The basic noun is fangelsi = prison.

A rough breakdown is:

• fangelsi = prison
• fangelsinu = in the prison / the prison (dative definite form)

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

So where English says the prison, Icelandic says fangelsið or, in this sentence’s case form, fangelsinu.

That is a normal feature of Icelandic grammar.

Instead of usually having a separate word like English the, Icelandic often puts the article onto the end of the noun.

For example:

• fangelsi = a prison / prison
• fangelsið = the prison

Because this sentence uses the dative after í for location, the form becomes fangelsinu.

So the article is still there, but it appears as part of the noun ending.

Because fangelsið and fangelsinu are different case forms.

• fangelsið is a form you would use when the noun is in the nominative or accusative definite singular, depending on context.
• fangelsinu is the dative definite singular form.

Since í expresses location here, it requires the dative, so fangelsinu is correct.

This is one of the most important patterns in Icelandic: the form of the noun changes depending on its role in the sentence.

Yes, and that is a very useful question.

Both can occur, but they are not always felt exactly the same.

• í fangelsi often means in prison in a more general, institutional sense.
• í fangelsinu more literally means in the prison or in the prison building / the specific prison.

In real usage, í fangelsi is very common when talking about someone being imprisoned as a general state, much like English He is in prison.

So a learner should notice this contrast:

• í fangelsi = in prison
• í fangelsinu = in the prison / in that prison

Context decides which sounds more natural.

This is the normal basic word order:

• Hann = subject
• er = verb
• í fangelsinu = prepositional phrase

So it follows a simple Subject–Verb–Other pattern, much like English:

• He is in prison.

Icelandic can move elements around more than English in some contexts, but this sentence is straightforward and neutral.

There are two main clues:

1. The preposition
   Í takes the dative when it means being located somewhere.

2. The ending
   The noun fangelsi is a neuter noun, and its definite dative singular form is fangelsinu.

So once you know the rule í + dative for location, the form makes sense.

A useful contrast is:

• Hann er í fangelsinu = He is in the prison.
• Hann fer í fangelsið = He goes into the prison.

In the second sentence, there is movement into the prison, so í takes the accusative instead.

A careful approximation is something like FANG-gel-si-nu, with the stress on the first syllable, because Icelandic words are usually stressed at the beginning.

A few helpful notes:

• g in Icelandic may sound softer than English learners expect, depending on position and speaker.
• -inu is pronounced clearly as part of the ending.
• The ll sound is not an issue here, unlike in some other Icelandic words.

If you are learning pronunciation, the most important thing here is:

• stress the first syllable: FANG-...
• pronounce all the syllables
• do not reduce endings too much, because endings carry grammar in Icelandic

The dictionary form is fangelsi.

That is the indefinite nominative singular form, which is the form you usually look up in a dictionary.

So if you see fangelsinu, you should learn to identify:

• base noun: fangelsi
• gender: neuter
• meaning: prison
• form in the sentence: definite dative singular

This is a very useful habit in Icelandic, since many noun forms look quite different from the dictionary form.