Glugginn hreyfist þegar vindurinn er sterkur.

In Icelandic, the definite article is usually attached to the end of the noun, not written as a separate word.

• gluggi = window

• glugginn = the window

• vindur = wind
• vindurinn = the wind

So the ending -inn here is basically the Icelandic way of saying the.

This is very common in Icelandic and is one of the first big differences English speakers notice.

Hreyfist is the present tense form of the verb hreyfast, which means to move.

In this sentence:

• Glugginn hreyfist = The window moves

It is 3rd person singular present, because the subject is glugginn (the window), which is singular.

So:

• ég hreyfist would be wrong here
• hann/hún/það hreyfist is the pattern this form matches

The -st ending is very common in Icelandic and often appears in what learners usually call the middle voice. In many cases, this gives a meaning like:

• something moves by itself
• something changes state
• something happens without a direct object

Here, hreyfast means to move, while hreyfa usually means to move something.

So the difference is roughly:

• Ég hreyfi gluggann. = I move the window.
• Glugginn hreyfist. = The window moves.

That is a very useful contrast to learn.

Because sterkur has to agree with vindurinn.

Here:

• vindurinn is masculine
• singular
• nominative

So the adjective is also:

• masculine
• singular
• nominative

That gives sterkur.

This is also because sterkur is a predicate adjective after the verb er (is):

• vindurinn er sterkur = the wind is strong

After vera (to be), Icelandic adjectives often stay in the nominative and agree with the subject.

Because each one is the subject of its own clause.

The sentence has two parts:

1. Glugginn hreyfist

   • glugginn is the subject of hreyfist

2. þegar vindurinn er sterkur

   • vindurinn is the subject of er

So both nouns are in the nominative because both are subjects.

Þegar means when here. It introduces a subordinate clause:

• þegar vindurinn er sterkur = when the wind is strong

So the whole sentence is built like this:

• main clause: Glugginn hreyfist
• subordinate clause: þegar vindurinn er sterkur

English speakers can think of þegar here as a normal time word introducing a when-clause.

Yes. Even though þegar literally means when, in a sentence like this the meaning is often general:

• The window moves when the wind is strong
• very naturally understood as
• The window moves whenever the wind is strong

The Icelandic present tense is often used for general facts or repeated situations, just like in English.

It can be rearranged.

The sentence as given is:

• Glugginn hreyfist þegar vindurinn er sterkur.

But Icelandic could also say:

• Þegar vindurinn er sterkur, hreyfist glugginn.

That version puts the when-clause first. Notice that in the main clause after that, the verb comes before the subject:

• hreyfist glugginn

This happens because Icelandic is a verb-second language in main clauses. So if something else comes first, the finite verb usually comes next.

Because Icelandic, like English, can simply repeat the noun instead:

• vindurinn er sterkur = the wind is strong

You could not naturally replace vindurinn with something like it unless the context had already clearly established the wind. In a simple standalone sentence, repeating vindurinn is normal and clear.

It is in the present tense.

• hreyfist = moves / is moving
• er = is

In a sentence like this, the present tense usually expresses a general present:

• The window moves when the wind is strong

It is not mainly about something happening right now at this exact second. It sounds more like a regular pattern or fact.

The most basic translation is moves, because hreyfast means to move.

But depending on context, English might sometimes express the idea more naturally as:

• shifts
• rattles
• shakes

For example, if a loose window reacts to strong wind, English might idiomatically say The window shakes when the wind is strong. But grammatically, the Icelandic word itself is still the general verb to move.

So moves is the safest core meaning, while other translations may fit better in specific situations.