Ég sagði henni að salurinn hefði verið hlýrri ef hurðin hefði ekki staðið opin svona lengi.

Because segja usually takes the person you tell in the dative: segja einhverjum eitthvað = tell someone something.

So:

• hún = she (nominative)
• henni = to her / her (dative)

In Ég sagði henni..., henni means to her.

Here að introduces a content clause, like English that:

• Ég sagði henni að... = I told her that...

In English, that is often omitted, but in Icelandic að is very common in this kind of sentence.

Because sagði refers to a real, completed act of speaking: I said / told. That part is presented as a fact, so the indicative is used.

The later clauses describe a hypothetical, unreal past situation, so Icelandic uses the subjunctive there:

• sagði = factual past
• hefði verið / hefði staðið = unreal or counterfactual past

So the contrast is very natural:

• real action: I told her
• unreal situation: the hall would have been warmer if...

Hefði verið hlýrri is the way Icelandic expresses a past counterfactual result here.

Breakdown:

• hefði = past subjunctive of hafa
• verið = past participle of vera (been)
• hlýrri = warmer

So the idea is:

• would have been warmer

Even though English uses would have been, Icelandic very often uses this subjunctive perfect/pluperfect-style construction instead.

Hlýrri is the comparative of hlýr:

• hlýr = warm
• hlýrri = warmer

It is used because the sentence compares the hall’s actual temperature with an unreal, better alternative:

• it would have been warmer

So hlýrri is not just saying the hall was warm; it says it would have been warmer than it actually was.

Because both clauses describe an unreal past situation.

• salurinn hefði verið hlýrri = the hall would have been warmer
• ef hurðin hefði ekki staðið opin = if the door had not been standing open

In English, the two parts usually look different:

• would have been
• had not been

In Icelandic, both often use subjunctive hefði in past counterfactual conditionals. That is normal and important to get used to.

Because Icelandic often uses standa opin(n) for doors, windows, and similar things being left open.

So:

• hurðin stóð opin = the door stood open / was standing open
• hurðin hafði staðið opin = the door had been standing open

This is more idiomatic than simply using vera in many cases. It emphasizes that the door remained in that open position.

Because opin agrees with hurðin, which is feminine singular.

The adjective is from opinn (open), and here it matches the noun:

• hurðin = the door (feminine singular)
• opin = open (feminine singular form)

So:

• hurðin hefði staðið opin = the door had stood open

This is ordinary adjective agreement.

Because Icelandic usually adds the definite article to the end of the noun.

So:

• salur = hall / room
• salurinn = the hall / room

and

• hurð = door
• hurðin = the door

This attached article is one of the basic differences from English.

After conjunctions like að and ef, Icelandic normally uses subordinate-clause word order.

So you get:

• að salurinn hefði verið hlýrri
• ef hurðin hefði ekki staðið opin

Notice the pattern:

• conjunction
• subject
• finite verb
• other elements

Also notice that ekki usually comes after the finite verb in this kind of neutral clause:

• hefði ekki staðið
• not ekki hefði staðið

That placement is very typical.

Svona lengi means something like this long or for so long.

• svona = like this / this way / this much
• lengi = long (in duration)

So opin svona lengi means:

• open this long
• open for so long

In natural English translation, for so long is often the best match.

Sometimes you may see myndi hafa verið, but in sentences like this, hefði verið is very normal and natural.

English often needs would have been, but Icelandic does not always mirror that with myndi. In past unreal conditions, the subjunctive hefði construction is extremely common.

So for a learner, the safest takeaway is:

• English would have been
• Icelandic often hefði verið

Do not expect Icelandic to copy English modal structure exactly.

Yes. It combines two things:

1. Reported speech / content clause

   • Ég sagði henni að...
   • I told her that...

2. A past unreal conditional

   • salurinn hefði verið hlýrri ef hurðin hefði ekki staðið opin svona lengi
   • the hall would have been warmer if the door had not been open so long

That combination is very common in real Icelandic, so this sentence is a useful model for advanced grammar.