Ef við hefðum ekki gist á tjaldstæðinu, hefðum við misst þetta fallega útsýni.

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

So the idea is:

• Ef við hefðum ekki gist á tjaldstæðinu = if we had not stayed overnight at the campsite
• hefðum við misst þetta fallega útsýni = we would have missed this beautiful view

In Icelandic, this kind of sentence typically uses the subjunctive with hafa plus a verb form like gist or misst.

Hefðum is the 1st person plural past subjunctive of hafa for við = we.

It appears in both clauses because both parts belong to the same unreal, hypothetical past situation:

• Ef við hefðum ekki gist ... = if we had not stayed
• hefðum við misst ... = we would have missed

So the Icelandic form is the same, but English translates it differently depending on the clause:

• in the if-clause: had
• in the main clause: would have

Gist comes from the verb gista, which means to stay overnight, to spend the night, or to lodge somewhere.

So gista is more specific than just stay. It usually implies an overnight stay.

In hefðum gist, gist is the verb form used after hafa in a perfect construction. For a learner, the easiest way to understand it is:

• gista = to stay overnight
• hefðum gist = had stayed overnight / would have stayed overnight

Misst comes from missa, which can mean to miss or to lose, depending on context.

Here it clearly means to miss in the sense of fail to see / fail to experience:

• hefðum við misst þetta fallega útsýni = we would have missed this beautiful view

Like gist, misst is the form used after hafa in this kind of construction.

There are two things going on here:

• tjaldstæði = campsite / camping ground
• tjaldstæðinu = the campsite
  The ending -inu is the definite article attached to the noun.

Also, the preposition á takes the dative here because this is a location, not motion toward somewhere.

So:

• á tjaldstæði = at a campsite
• á tjaldstæðinu = at the campsite

This is very natural Icelandic with gista.

This is because Icelandic main clauses usually follow a verb-second pattern.

The sentence begins with the if-clause:

• Ef við hefðum ekki gist á tjaldstæðinu

Once that whole clause is placed first, the main clause begins, and the finite verb comes next:

• hefðum við misst þetta fallega útsýni

So the order is natural because the first position is already taken by the conditional clause.

Compare:

• Við hefðum misst þetta fallega útsýni. = We would have missed this beautiful view.
• Ef ..., hefðum við misst þetta fallega útsýni. = If ..., we would have missed this beautiful view.

In this kind of verb phrase, ekki usually comes after the finite verb and before the non-finite verb form.

So:

• við hefðum ekki gist = we had not stayed overnight

This is the normal placement:

• finite auxiliary: hefðum
• negation: ekki
• main verb form: gist

The same pattern appears very often in Icelandic.

Because after a word like þetta = this, the adjective normally takes the weak form.

So:

• fallegt útsýni = a beautiful view
• þetta fallega útsýni = this beautiful view

Here:

• þetta = this
• fallega = beautiful, in the weak form
• útsýni = view

This is a very common pattern in Icelandic:

• gott hús = a good house
• þetta góða hús = this good house

Because þetta fallega útsýni is the direct object of misst.

The verb missa takes the accusative case, so the object has to be in the accusative.

Since útsýni is neuter singular, the accusative form of the demonstrative is þetta.

So:

• misst hvað? = missed what?
• þetta fallega útsýni

Þessu would be dative, which is not what the verb needs here.

Yes. Gista is more specific and idiomatic here.

• vera just means to be
• gista means to stay overnight / spend the night

So gist á tjaldstæðinu tells you not only where they were, but that they spent the night there, which fits the campsite context very naturally.

That makes the sentence more precise than simply saying they were at the campsite.