Hliðið er opið.

Because -ið is the definite article attached to the noun.

• hlið = gate / a gate
• hliðið = the gate

Icelandic often puts the at the end of the noun instead of using a separate word like English does.

So in this sentence, hliðið means the gate, not just gate.

The dictionary form is hlið.

That is the indefinite singular form. When you add the definite article, it becomes hliðið.

So:

• hlið = gate
• hliðið = the gate

Also, hlið is a neuter noun, and that matters for the adjective later in the sentence.

Because adjectives in Icelandic must agree with the noun they describe.

The noun hlið is:

• neuter
• singular
• here, nominative

So the adjective also has to be neuter singular nominative, which gives:

• masculine: opinn
• feminine: opin
• neuter: opið

Since hliðið is neuter singular, the correct form is opið.

The dictionary form is opinn, meaning open.

In Icelandic, adjectives are usually listed in the masculine singular nominative form, so you look up opinn, not opið.

The forms are:

• opinn = masculine
• opin = feminine
• opið = neuter

So opið is just the neuter form of opinn.

Because after vera (to be), Icelandic normally uses the strong predicate form of the adjective, not a special weak form just because the noun is definite.

So even though hliðið means the gate, you still say:

• Hliðið er opið.

not a weak-form version.

This is very normal in Icelandic: predicate adjectives usually agree in gender, number, and case, but they are still often in the strong form.

Er is the present tense of vera, which means to be.

So:

• er = is

That makes the sentence a simple statement:

• Hliðið er opið = The gate is open

Unlike in some languages, you normally cannot leave out the verb to be in a sentence like this in Icelandic.

Here it is in the nominative case, because it is the subject of the sentence.

The sentence structure is:

• Hliðið = the subject
• er = the verb
• opið = a predicate adjective describing the subject

So hliðið is nominative, and opið matches it.

One small extra point: for many neuter nouns, the nominative and accusative forms are the same, so the form itself does not always tell you the case. In this sentence, the grammar tells you it is nominative.

Here it means open as a state or condition.

So Hliðið er opið means the gate is in an open state.

It does not mean the gate was opened as an action. If you wanted the action, Icelandic would use a different structure, for example with a passive form such as:

• Hliðið var opnað = The gate was opened

So:

• opið = open
• opnað = opened / was opened in a passive construction

That is an important distinction.

A few key pronunciation points:

• Stress is on the first syllable of each word.
• hl at the start of hliðið is not pronounced like a full English h plus l. It is more like a breathy/devoiced l sound.
• ð is like the th in this, not the th in thin.
• er is usually short and light.

A rough English-friendly guide would be something like:

HLI-thith er OH-pith

But that is only approximate. The main sounds to notice are the special hl and the voiced ð.

Yes, but Hliðið er opið is the normal, neutral order.

Icelandic allows some flexibility, and because of verb-second patterns, you can move things around for emphasis. For example, a marked version like:

• Opið er hliðið

is possible in some contexts, but it sounds more literary, poetic, or contrastive.

For everyday speech, the standard order is:

• Hliðið er opið

It means gate, not the usual word for door.

The common Icelandic word for door is:

• hurð

So:

• Hliðið er opið = The gate is open
• Hurðin er opin = The door is open

This is also a useful comparison, because hurð is feminine, so the adjective changes to opin instead of opið.