Netnámskeiðið er skemmtilegra en ég hélt.

Because Icelandic usually writes compound nouns as a single word.

Here, netnámskeiðið is built from:

• net = internet / online
• námskeið = course
• -ið = the

So the whole word means the online course. English often uses several separate words where Icelandic prefers one compound word.

-ið is the definite article, meaning the.

In Icelandic, the is usually attached to the end of the noun instead of being a separate word.

• námskeið = course
• námskeiðið = the course

Since námskeið is a neuter noun, -ið is the correct definite ending here.

It is neuter singular nominative.

Why?

• The base noun námskeið is neuter
• It is singular
• It is the subject of er, so it appears in the nominative

That matters because the adjective skemmtilegra has to match it.

Because netnámskeiðið is neuter singular, and the comparative adjective has to agree with it.

The basic adjective is:

• skemmtilegur = masculine
• skemmtileg = feminine
• skemmtilegt = neuter

In the comparative:

• skemmtilegri is used for masculine/feminine singular in forms like the nominative
• skemmtilegra is the neuter singular form

So with netnámskeiðið (a neuter noun), skemmtilegra is the correct form.

Not exactly in a word-for-word sense, but it is the natural Icelandic adjective for fun, enjoyable, or entertaining.

So:

• skemmtilegt námskeið = a fun / enjoyable course
• skemmtilegra = more fun / more enjoyable

English uses fun very freely as an adjective, while Icelandic usually uses skemmtilegur in this kind of sentence.

Here en means than.

It is used after a comparative:

• skemmtilegra en... = more fun than...

So this works just like English:

• better than
• more interesting than
• more fun than

That is a very common question.

The verb halda does often mean hold, but it also has another common meaning: think, believe, or suppose.

So:

• ég held = I think / I believe
• ég hélt = I thought / I believed

This verb is irregular, so the past tense changes the vowel:

• held
• hélt

In this sentence, ég hélt means I thought or I assumed.

Because the speaker is comparing reality now with an earlier expectation.

So the idea is:

• It is more fun than I thought

That is the same logic as in English. The thought happened earlier, so Icelandic uses the past tense hélt.

If you said en ég held, that would mean than I think, which is a different idea.

Yes, you can think of the sentence as slightly shortened.

A fuller version might be something like:

• Netnámskeiðið er skemmtilegra en ég hélt að það væri
• Netnámskeiðið er skemmtilegra en ég hélt að það yrði

In English we do the same thing:

• It is more fun than I thought
• not necessarily ...than I thought it would be

So en ég hélt is perfectly natural, and the missing part is understood from context.

A useful breakdown is:

• Netnámskeiðið = the online course
• er = is
• skemmtilegra = more fun / more enjoyable
• en = than
• ég hélt = I thought

So the structure is:

The-online-course is more-fun than I-thought.

The word order is very close to English, which makes this sentence fairly beginner-friendly once you understand the adjective form and hélt.