Það er langt síðan dyrasíminn virkaði svona vel.

It is a very common Icelandic time expression meaning It has been a long time since ...

So:

Það er langt síðan dyrasíminn virkaði svona vel.

means the same idea as It’s been a long time since the intercom worked this well.

Grammatically, the pattern is:

Það er langt síðan + past clause

You should learn það er langt síðan as a set phrase, because it does not translate word-for-word in a perfectly natural way.

Here það is a dummy or expletive subject, like English it in sentences such as It is raining or It has been a long time.

It does not point to a specific thing. Icelandic often uses það in this kind of impersonal expression just because the sentence needs a grammatical subject.

So in this sentence, það is there for structure, not for concrete meaning.

Langt is the form normally used in this expression: það er langt síðan ...

A useful way to think of it is that Icelandic often uses the neuter singular form of an adjective adverbially. Here langt works a bit like English long in long since.

Why not the others?

• langur would be wrong here because nothing masculine singular is being described.
• lengi is another adverb meaning for a long time, but this specific construction normally uses langt.

So the natural phrase is:

Það er langt síðan ...

and not Það er langur síðan ...

After síðan in this kind of sentence, Icelandic normally uses the past tense.

So virkaði means worked / was functioning and refers to the earlier time point being compared with now.

English often uses either:

• It’s been a long time since it worked this well
• or sometimes a present-perfect style idea

But Icelandic usually just uses the simple past in the clause after síðan:

• Það er langt síðan ég sá hana.
• Það er langt síðan tölvan virkaði almennilega.

So virkaði is exactly what you would expect here.

Dyrasíminn is the definite form of dyrasími, which means intercom or door phone.

It breaks down like this:

• dyrasími = intercom
• dyrasíminn = the intercom

The ending -inn is the suffixed definite article, which is very normal in Icelandic. Instead of putting a separate word like the before the noun, Icelandic often adds the definiteness to the end of the noun.

In this sentence, dyrasíminn is the subject of virkaði, so it is in the nominative singular.

Svona means like this, this way, or like that in the sense of manner.
Vel means well.

So svona vel means this well or as well as this.

The idea is that the speaker is comparing the present situation with the past:

It hasn’t worked this well in a long time.

That is why svona is very natural here: it points to the current level of functioning.

Sometimes svo vel can be possible in Icelandic, but it has a slightly different feel.

• svona vel = this well, comparing with how things are now
• svo vel = more like so well, focusing on degree

In this sentence, svona vel is the more natural choice because the speaker is basically saying:

It hasn’t worked as well as it is working now for a long time.

So for this exact meaning, svona vel fits better.

That is a very good question. Icelandic does have verb-second word order in main clauses, but the part after síðan is a subordinate clause.

In subordinate clauses, you normally do not get the same main-clause verb-second pattern. So the normal order is:

subject + verb

That is why you get:

síðan dyrasíminn virkaði svona vel

and not something like a main-clause inversion pattern.

So the sentence has:

• main clause: Það er langt síðan
• subordinate clause: dyrasíminn virkaði svona vel

No. Síðan can have more than one use in Icelandic.

In this sentence, it functions as a conjunction meaning since, introducing the clause dyrasíminn virkaði svona vel.

But síðan can also be used as an adverb meaning something like after that, then, or since then, depending on context.

So learners often meet síðan in several different roles. Here, though, the important one is the conjunction in the pattern:

Það er langt síðan ...

Yes, absolutely. This is a very productive pattern.

For example:

Það er stutt síðan ég kom heim.
It hasn’t been long since I came home.

Það eru tvö ár síðan við fluttum.
It has been two years since we moved.

Það er langt síðan hún skrifaði mér.
It’s been a long time since she wrote to me.

Notice that if you use a plural time expression like tvö ár, Icelandic uses plural eru:

• Það eru tvö ár síðan ...

But with langt, stutt, and similar expressions, you usually get singular er:

• Það er langt síðan ...
• Það er stutt síðan ...