Glugginn festist þegar ég reyndi að opna hann.

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

• gluggi = a window
• glugginn = the window

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

Also notice that gluggi changes slightly to glugginn:

• base form: gluggi
• definite nominative singular: glugginn

That ending is very common with masculine nouns.

Because gluggi is a masculine noun in Icelandic.

In Icelandic, nouns have grammatical gender:

• masculine
• feminine
• neuter

Since gluggi is masculine, you refer back to it with hann.

So here:

• glugginn = the window
• hann = it referring to that masculine noun

English uses it for things, but Icelandic often uses hann, hún, or það depending on the noun’s grammatical gender, not biological sex.

Festist means something like got stuck or jammed in this sentence.

It comes from the verb festast, which often means:

• to get stuck
• to become stuck
• to jam

So Glugginn festist means the window was fine before, and then it became stuck at that moment.

That is a little different from simply describing a state. For example:

• Glugginn var fastur = The window was stuck
  This describes the condition.

• Glugginn festist = The window got stuck
  This focuses on the change or event.

The -st ending is very common in Icelandic and can do several jobs. In this sentence, it helps create the idea of something happening to itself or becoming a certain way.

With festast, the sense is not usually that someone actively sticks something, but that it gets stuck.

Compare:

• festa = to fasten, attach
• festast = to get stuck, become stuck

So the -st form often makes the meaning less directly active and more like:

• reflexive
• middle voice
• change of state
• sometimes passive-like

You do not always translate -st the same way in English, so it is best to learn each verb as a vocabulary item.

Þegar means when here, introducing a time clause.

So:

• þegar = when
• ég reyndi = I tried

Together:

• þegar ég reyndi að opna hann = when I tried to open it

This is a very normal way to connect two past events in Icelandic.

Also notice that the verb in the þegar clause is in the past tense because the whole sentence is talking about a past situation.

Because reyna is followed by að + infinitive when you mean try to do something.

So the pattern is:

• reyna að + infinitive

Examples:

• reyna að sofa = try to sleep
• reyna að skilja = try to understand
• reyna að opna = try to open

In your sentence:

• reyndi = past tense of reyna
• að opna = to open

So ég reyndi að opna hann literally follows the standard Icelandic structure for I tried to open it.

Because opna takes a direct object, and that object is in the accusative case.

The pronoun forms are:

• nominative: hann
• accusative: hann
• dative: honum
• genitive: hans

Here the window is the thing being opened, so it is the direct object:

• að opna hann = to open it

You would use honum only if the verb required the dative, which opna does not here.

In this sentence, hann is needed because að opna needs an object: open what?

• ég reyndi að opna = I tried to open ...
• ég reyndi að opna hann = I tried to open it

So hann is not just optional repetition. It completes the meaning of the verb.

Icelandic does sometimes leave things implied from context, but in a normal sentence like this, keeping hann is the natural choice.

The basic structure is:

• Glugginn festist = main clause
• þegar ég reyndi að opna hann = subordinate time clause

So the full sentence is:

• Glugginn festist
• þegar ég reyndi að opna hann

A useful thing to notice is that the subordinate clause introduced by þegar has normal clause order:

• ég reyndi not
• reyndi ég

That is because þegar introduces a dependent clause.

If you started with the þegar clause, Icelandic would then show normal main-clause verb-second behavior in the next clause:

• Þegar ég reyndi að opna hann, festist glugginn.

That version is also natural.

Yes, depending on context, þegar can overlap with English when and sometimes as/while.

In this sentence, the usual sense is:

• the window got stuck at the time I tried to open it

English might express that as:

• when I tried to open it
• as I tried to open it
• sometimes even while I was trying to open it

But the most straightforward match is still when.

If you wanted to strongly emphasize an ongoing action in Icelandic, you might choose a different structure, but þegar is perfectly normal here.

Reyndi is the past tense of reyna.

The infinitive is:

• reyna = to try

The past tense for ég is:

• ég reyndi = I tried

This is an irregular-looking form from the learner’s point of view because the stem changes:

• reyna
• reyndi

So it is a good idea to learn it as a full verb pattern rather than trying to build it mechanically every time.

Yes, but it means something slightly different.

• Glugginn festist... = The window got stuck...
  This suggests a change happened.

• Glugginn var fastur... = The window was stuck...
  This describes the state of the window.

So if the idea is that your attempt caused or coincided with the window becoming jammed, festist is better.

If you simply mean the window was already stuck when you tried, var fastur may be better.

Yes, a few common ones:

• glugginn: the gg is pronounced as a true double consonant, and the -inn ending is clear.
• festist: the -st ending is very common and worth practicing as one cluster.
• þegar: þ is like English th in thin, not like t.
• ég: usually pronounced like yeg with a clear j/y sound at the start.
• reyndi: the ey is a diphthong, and the cluster -ynd- can take practice.
• opna: the pn cluster is normal in Icelandic and may feel unusual to English speakers.

A slow breakdown might help:

• Glugginn
• festist
• þegar
• ég reyndi
• að opna hann

Practicing the sentence in chunks is often easier than saying it all at once.