Ef tjaldið hefði verið stærra, hefðum við sofið betur um nóttina.

It is a counterfactual conditional: a sentence about something that did not actually happen in the past.

The idea is:

• the tent was not bigger
• because of that, we did not sleep better

So Icelandic is expressing the same kind of idea as English If the tent had been bigger, we would have slept better during the night.

They are both subjunctive past forms of hafa.

• hefði = singular past subjunctive
• hefðum = plural past subjunctive

In this kind of unreal past conditional, Icelandic commonly uses:

• ef
  • past subjunctive in the if-clause
• past subjunctive in the main clause as well

So:

• Ef tjaldið hefði verið stærra = If the tent had been bigger
• hefðum við sofið betur = we would have slept better

English uses would have, but Icelandic does not need a separate word exactly like would here. The subjunctive does that job.

They are the forms used after hafa to make a perfect construction.

• hefði verið = had been
• hefðum sofið = would have slept / had slept depending on context

The dictionary forms are:

• vera = to be
• sofa = to sleep

After hafa, Icelandic uses the supine form:

• vera → verið
• sofa → sofið

So even though English learners may think of these as past participles, in Icelandic grammar it is more precise to call them the supine in this construction.

Because stærra is the correct form of the comparative adjective agreeing with tjaldið.

Here is what is happening:

• stór = big
• stærri = bigger, but not the form used for a neuter singular subject here
• stærra = the neuter singular comparative form

Since tjaldið is a neuter singular noun, the adjective also has to be neuter singular:

• tjaldið = the tent
• tjaldið ... stærra = the tent ... bigger

So stærra agrees grammatically with tjaldið.

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

• tjald = tent
• tjaldið = the tent

In Icelandic, the definite article is usually added to the end of the noun rather than written as a separate word.

So tjaldið literally means tent-the, but in normal English we just say the tent.

Because Icelandic is a verb-second language.

When the sentence begins with the if-clause:

• Ef tjaldið hefði verið stærra

that whole clause takes the first position. Then the finite verb of the main clause comes next:

• hefðum

and only after that comes the subject:

• við

So the pattern is:

• [fronted clause] + [finite verb] + [subject]

That is why you get:

• Ef tjaldið hefði verið stærra, hefðum við sofið betur ...

If the main clause stood by itself, you could say:

• Við hefðum sofið betur um nóttina.

But after the initial ef-clause, the verb moves before the subject.

Because betur is the comparative form of the adverb vel.

• vel = well
• betur = better

Since the sentence is talking about how we would have slept, Icelandic needs an adverb, not an adjective.

So:

• sofa vel = sleep well
• sofa betur = sleep better

That is exactly parallel to English.

Here it means during the night or at night.

• um
  • a time expression can mean during
• nóttina is the accusative form of nóttin = the night

So:

• um nóttina = during the night / through the night

In this sentence it refers to the night in question: if the tent had been bigger, we would have slept better during that night.

Because the preposition um takes the accusative in this kind of time expression.

So the noun nótt appears in the accusative definite form:

• nominative definite: nóttin = the night
• accusative definite: nóttina

That is why the sentence has:

• um nóttina

A learner coming from English often expects the noun just to stay the same, but in Icelandic the preposition controls the case.

No. Ef can be followed by either indicative or subjunctive, depending on meaning.

A simple way to think about it:

• indicative: real, open, or possible condition
• subjunctive: unreal, hypothetical, or counterfactual condition

In this sentence, the condition is unreal in the past, so the subjunctive is used:

• Ef tjaldið hefði verið stærra ...

But for a real possibility, Icelandic could use indicative, for example the equivalent of:

• If the tent is bigger, we sleep better

So the subjunctive here is not because of ef alone; it is because of the counterfactual meaning.

Yes, you can think of þá as being implicitly understood, even though it is not necessary.

The sentence could be expanded to something like:

• Ef tjaldið hefði verið stærra, þá hefðum við sofið betur um nóttina.

Here þá means something like then.

In many Icelandic sentences, especially in everyday usage, þá is simply omitted because the meaning is already clear. So the version without þá is completely natural.

Then Icelandic would normally use a different pattern, for example:

• Ef tjaldið væri stærra, myndum við sofa betur um nóttina.

That means something like:

• If the tent were bigger, we would sleep better at night / during the night

Compare the two:

• Ef tjaldið hefði verið stærra, hefðum við sofið betur um nóttina.
  = past unreal: the tent was not bigger, and we did not sleep better

• Ef tjaldið væri stærra, myndum við sofa betur um nóttina.
  = present unreal: the tent is not bigger now, and we would sleep better if it were

So the original sentence is specifically about a past missed possibility.