Fangelsið er stórt.

The ending -ið is the suffixed definite article in Icelandic. Instead of using a separate word for the, Icelandic often adds it to the end of the noun.

So:

• fangelsi = prison
• fangelsið = the prison

This is very common in Icelandic.

The dictionary form is fangelsi, meaning prison.

Fangelsið is the same noun with the definite article attached. So if you look it up in a dictionary, you would usually search for fangelsi, not fangelsið.

Because Icelandic usually expresses the by attaching it to the noun itself.

Compare:

• English: the prison
• Icelandic: fangelsið

This is one of the first big differences English speakers notice. Icelandic can use a separate demonstrative in some contexts, but for ordinary the, the article is typically attached to the noun.

Er means is.

It is the present tense singular form of the verb vera (to be).

So in this sentence:

• fangelsið = the prison
• er = is
• stórt = big / large

Because the adjective has to agree with the noun in gender, number, and case.

The noun fangelsi is neuter singular, so the adjective must also be in the neuter singular form:

• masculine: stór
• feminine: stór
• neuter: stórt

Since fangelsið is neuter, Icelandic uses stórt.

You usually learn the gender together with the noun. In dictionaries, Icelandic nouns are often marked as:

• m. = masculine
• f. = feminine
• n. = neuter

Fangelsi is a neuter noun.

Also, many neuter nouns have forms that can help you recognize the pattern, but in general it is safest to memorize the gender when you learn each noun.

Because Icelandic can simply use the noun itself as the subject:

• Fangelsið er stórt. = The prison is big.

There is no need for a separate dummy subject like English it. The subject is already fangelsið.

This sentence uses the most basic Icelandic word order:

• subject + verb + adjective

So:

• Fangelsið = subject
• er = verb
• stórt = predicate adjective

This is very similar to English The prison is big.

Usually, no—not in normal natural Icelandic for this meaning.

Fangelsi without -ið is prison or a prison, but Icelandic does not use the indefinite article a/an the way English does. A bare noun can sometimes be used in general statements, but if you mean the prison in a specific sentence, fangelsið is the natural form.

So:

• fangelsi = prison / a prison depending on context
• fangelsið = the prison

You would say:

• Fangelsið er lítið.

Here the same agreement rule applies. Since fangelsið is neuter singular, the adjective must also be neuter singular:

• lítill = masculine
• lítil = feminine
• lítið = neuter

A rough learner-friendly pronunciation is:

• fangelsið ≈ FANG-gel-sith
• er ≈ ehr
• stórt ≈ stohrt

A few notes:

• The ð in Icelandic is often like the th in this, though pronunciation can vary by position.
• The ó is a long vowel, roughly like o in more, but not exactly the same as English.
• The final -t in stórt is clearly pronounced.

Exact Icelandic pronunciation takes practice, but this approximation can help you get started.

Yes. Stórt can be translated as big or large, depending on context and style.

So Fangelsið er stórt. could naturally be understood as either:

• The prison is big.
• The prison is large.

It is in the nominative singular.

That is because it is the subject of the sentence. In a simple equational sentence like this, the subject is normally nominative:

• Fangelsið = nominative singular
• er = is
• stórt = adjective agreeing with the neuter singular subject

So this sentence is a good basic example of nominative subject + vera + adjective agreement.