Ef ég hefði haft þurrari gönguskó, hefði ég setið lengur við fossinn.

It is a past counterfactual conditional: it talks about something that did not actually happen.

English uses the same idea in If I had had..., I would have...

So Icelandic Ef ég hefði haft..., hefði ég setið... is the pattern for:

• an unreal past condition
• and its unreal past result

Because Icelandic commonly uses hefði + supine in both parts of a past unreal conditional.

Here:

• hefði haft = had had
• hefði setið = would have sat

So the structure is:

• Ef
  • subject + hefði
    • verb form
• main clause with hefði
  • verb form

This is very normal in Icelandic for sentences about imagined past situations.

They are the verb forms used after the auxiliary hafa in perfect-type constructions. In learner-friendly terms, you can think of them as past-participle-like forms; more precisely, they are often called the supine.

So:

• hafa → haft
• sitja → setið

You also see them in ordinary perfects:

• Ég hef haft... = I have had...
• Ég hef setið... = I have sat / been sitting...

In this sentence, the past subjunctive auxiliary hefði combines with those forms.

Because Icelandic is basically a verb-second language in main clauses.

The whole Ef... clause comes first, and that takes the first slot. Then the finite verb of the main clause must come next:

• Ef ég hefði haft þurrari gönguskó, hefði ég setið lengur við fossinn.

If you put the main clause first, the order changes back:

• Ég hefði setið lengur við fossinn ef ég hefði haft þurrari gönguskó.

So hefði ég here is a word-order effect, not a different meaning.

Þurrari is the comparative form of þurr = dry.

So:

• þurr = dry
• þurrari = drier

The comparison is implicit: drier than the hiking boots I actually had.

This is just how Icelandic forms many comparatives: with a special comparative adjective form rather than a separate word like more.

Because it is the object of haft, and that object is in the accusative.

The dictionary form is:

• gönguskór = hiking boot / hiking boots

But after haft, you need the accusative form:

• gönguskó

A useful thing to know is that skór is irregular, and some singular/plural forms look the same. Here, context makes it clear that gönguskó means hiking boots.

Because lengur is an adverb, while lengri is an adjective.

In this sentence, it modifies the verb setið:

• setið lengur = sat longer / sat for longer

Compare:

• lengri tími = a longer time
  Here lengri is an adjective describing a noun.

So:

• lengur = longer, for longer (adverb)
• lengri = longer (adjective before a noun)

Because við is the natural preposition here for by / at / next to, and it takes the accusative.

So:

• við fossinn = by the waterfall

And fossinn breaks down as:

• foss = waterfall
• -inn = the suffixed definite article

So fossinn means the waterfall.

Because the noun is indefinite here.

The sentence means drier hiking boots, not the drier hiking boots.

In Icelandic, definiteness is usually added as a suffix on the noun. If you wanted the hiking boots, you would use a definite form such as gönguskóna in the accusative plural.

So the contrast is roughly:

• þurrari gönguskó = drier hiking boots
• þurrari gönguskóna = the drier hiking boots

The indefinite version is the natural one in this sentence.

No. The original sentence is good, but there are other natural options depending on what you want to emphasize.

For example:

• Ef ég hefði verið í þurrari gönguskóm, hefði ég setið lengur við fossinn.
  = If I had been wearing drier hiking boots, I would have sat longer by the waterfall.

• Ef ég hefði verið með þurrari gönguskó, hefði ég setið lengur við fossinn.
  = a more colloquial if I had had / been with drier hiking boots

So:

• haft emphasizes having
• verið með can sound more conversational
• verið í emphasizes wearing

All three can be useful, but they are not exactly identical in nuance.