Learn Hardcore Icelandic: Það er erfitt að læra íslensku, en það er gaman. - It is difficult to learn Icelandic, but it is fun.

Það er erfitt að læra íslensku literally: It is difficult to learn Icelandic. The real idea is að læra íslensku (to learn Icelandic), but Icelandic prefers to put það in subject position and move the að-clause after the adjective.
The second það in en það er gaman again works like it: but it is fun (meaning: but learning Icelandic is fun).

You normally cannot drop það here; *… en er gaman is ungrammatical as a full sentence. In informal speech people might just say … en gaman! as a fragment, but in a complete sentence það is expected.

Why is it erfitt and not erfiður or erfið?

The dictionary form of the adjective is usually erfiður (masculine). Icelandic adjectives change form to agree with the gender, number, and case of what they describe.

The basic forms are:

Masculine: erfiður
Feminine: erfið
Neuter: erfitt

In Það er erfitt að læra íslensku, the adjective is not describing a masculine or feminine noun like maður (man) or bók (book). Instead, it describes the general fact að læra íslensku (to learn Icelandic). Icelandic treats this kind of abstract “it/that” as neuter singular, so the adjective is erfitt.

What exactly is að doing in að læra? Is it the same as English to in to learn?

Yes. Here að is the infinitive marker, very similar to English to before a verb:

að læra = to learn
að borða = to eat
að lesa = to read

This að is different from:

að as a preposition (often meaning to, at, towards), and
að as a subordinating conjunction (like English that in I know that…).

You usually keep að before an infinitive unless a verb that comes before specifically drops it (for example: ég vil læra íslensku – I want to learn Icelandic, no að after vil).

Why is it íslensku and not íslenska in að læra íslensku?

Íslenska is a feminine noun meaning Icelandic (the Icelandic language). Icelandic nouns change form depending on case.

For a regular feminine -a noun (like íslenska), the singular forms are:

Nominative: íslenska (used for the subject)
Accusative: íslensku
Dative: íslensku
Genitive: íslensku

In að læra íslensku, íslensku is the direct object of læra, so it goes into the accusative case. For this noun, the accusative singular is íslensku, not íslenska.

Compare:

Íslenska er fallegt tungumál. – Icelandic is a beautiful language. (subject → nominative íslenska)
Ég læri íslensku. – I am learning Icelandic. (object → accusative íslensku)

Could I also say Að læra íslensku er erfitt, en það er gaman? Is that just a word‑order change?

Yes, that is correct Icelandic, and it is mostly a word‑order / focus difference.

Það er erfitt að læra íslensku… is the most neutral English‑like pattern: It is hard to learn Icelandic…
Að læra íslensku er erfitt… puts “learning Icelandic” explicitly in subject position: Learning Icelandic is hard…

Both are grammatically fine. The version with það is a bit more common and stylistically neutral; the version with Að læra íslensku first puts more focus on the activity itself.

What is gaman exactly? Is it an adjective like fun, or a noun?

Historically, gaman is a neuter noun meaning fun, enjoyment, amusement, but in modern Icelandic it behaves a lot like a fixed predicate word, almost like an adjective in patterns such as:

Það er gaman. – It is fun.
Það er gaman að læra íslensku. – It is fun to learn Icelandic.
Mér finnst gaman. – I find it fun / I enjoy it.

Some points:

It is neuter singular and in this use it doesn’t change form (*gömum etc. do not exist).
You normally don’t say *mjög gamanlegt here; gaman itself carries the idea of fun.
In your sentence, en það er gaman means but it is fun, with the activity að læra íslensku understood from the first clause.

Why don’t we repeat að læra íslensku after gaman? Is it okay to leave it out?

Yes, it’s natural to leave it out because it is obvious from context. This is called ellipsis: leaving out something that can be easily understood.

So:

Full version: Það er erfitt að læra íslensku, en það er gaman að læra íslensku.
Natural shortened version: Það er erfitt að læra íslensku, en það er gaman.

The second það er gaman is understood as “it (learning Icelandic) is fun”. Repeating að læra íslensku would sound heavy and a bit unnatural unless you really want to emphasize it.

Is the second það in en það er gaman really necessary? Could I just say … en gaman?

In a full, normal sentence, you keep það:

…, en það er gaman. – …, but it is fun.

Dropping it to say *…, en er gaman is not grammatical. Icelandic generally does not drop subject pronouns the way some other languages do.

What you might hear in spontaneous speech is a fragment, for example:

Erfitt að læra íslensku, en gaman! – Hard to learn Icelandic, but fun!

But that is a shortened, informal style. When writing or speaking in full sentences, use það er gaman.

What does en mean here, and why is there a comma before it?

Here en is a coordinating conjunction meaning but. It connects two main clauses:

Það er erfitt að læra íslensku,
en það er gaman.

The comma before en is normal in Icelandic when you join two independent clauses:

Ég er þreyttur, en ég þarf að læra. – I am tired, but I have to study.

Modern style guides differ a bit on how strictly you must use that comma, but in practice, writing …, en … between two main clauses is very common and perfectly correct.

How would I make this sentence negative, like It is not difficult to learn Icelandic, but it is fun?

You add ekki (not) after the verb er in the first clause:

Það er ekki erfitt að læra íslensku, en það er gaman.
- er ekki erfitt – is not difficult

Word order around ekki is quite strict:

Það er erfitt. → Það er ekki erfitt.
Það er gaman. → Það er ekki gaman.

So you cannot say *Það ekki er erfitt or move ekki away from er in this simple pattern.

How do I pronounce það, erfitt, and íslensku, especially the letters þ, ð, and í?

A rough guide for English speakers:

þ is like unvoiced th in think:
- það starts with that sound: [θaːð] (roughly thahth; the final ð is soft and may be barely audible).
ð is like voiced th in this, but often very soft, especially at the end of a word.
í is a long i‑sound, like ee in see, but kept pure (no glide to y):
- ís‑ in íslensku sounds like ees.
erfitt: stress on the first syllable: ÉR‑fitt. The tt indicates a kind of strong t with a little breath before it; as a learner you can just say a clear short t at the end.
íslensku: ÍS‑len‑sku, with stress on the first syllable (ÍS‑). Every Icelandic word has main stress on the first syllable.

Why is there no article like the in íslensku? In English we might say to learn Icelandic or to learn the Icelandic language.

Icelandic generally does not use an article with language names in this kind of sentence.

að læra íslensku – to learn Icelandic
að tala íslensku – to speak Icelandic
ég kann íslensku – I know Icelandic

If you explicitly say the Icelandic language you still usually skip the article:

íslenska tungumálið – the Icelandic language (here the definite form tungumálið carries the definiteness; you don’t add a separate the‑word).

So að læra íslensku is the normal way to say to learn Icelandic, without any extra article word.