Learn Hardcore Icelandic: Ég sef lengur, því það er frídagur á morgun. - I sleep longer, because it is a day off tomorrow.

So → se is a regular strong-verb vowel change in Icelandic. Many strong verbs change the stem vowel between infinitive and present (and again in the past). So sofa – sef is just a pattern you have to memorize, similar to English sleep – slept (vowel change) but in different forms.

What is the difference between lengi and lengur?

Both are related to long (time):

lengi = for a long time
lengur = for a longer time / longer (any more) – the comparative form

In the sentence Ég sef lengur, lengur means “longer (than usual)” or “for a longer time”, and often translates naturally as “I sleep in” rather than literally I sleep longer.

Example contrast:

Ég sef lengi – I sleep for a long time.
Ég sef lengur – I sleep longer / I will sleep in.

Why is there a comma before því? Is this always required?

In Icelandic, it is normal (and often recommended) to put a comma before því when it means because and introduces a reason clause:

Ég sef lengur, því það er frídagur á morgun.

This comma separates the main clause (Ég sef lengur) from the reason clause (því það er frídagur á morgun).

In modern less-formal writing, some people may omit such commas more often, but you will very commonly see and are safe using the comma here.

Does því always mean “because”? I’ve seen it used differently.

No, því has several functions in Icelandic, which can be confusing:

As “because” (conjunction/adverbial):
- Ég sef lengur, því það er frídagur á morgun.
  Here it means “because”.
As part of the phrase “af því að” = “because”:
- Ég sef lengur, af því að það er frídagur á morgun.
  This is another common and often more explicit way to say because.
As a dative form of “það” (it/that) in other contexts, often translated as “because of that / for that reason / therefore”:
- Ég sef lengur. Því þarf ég vekjaraklukku.
  (Literally: For that reason I need an alarm clock.)

So in your sentence, því is best understood simply as “because”, but be aware that in other contexts it can function differently.

Could I also say “Ég sef lengur, af því að það er frídagur á morgun”? Is there a difference?

Yes, that sentence is correct:

Ég sef lengur, af því að það er frídagur á morgun.

This is a very common and clear way to say “because”. Nuances:

því alone: a bit shorter, can feel a little more written or stylistic in some registers, but perfectly normal.
af því að: very common in everyday speech and writing, explicitly because.

In this context, both versions mean the same thing:
I’ll sleep longer because it’s a holiday tomorrow.

Why is it á morgun for “tomorrow”? Doesn’t á mean “on”?

Literally, á morgun is “on the morning” or “in the morning”, but idiomatically it has come to mean “tomorrow”.

morgun = morning
á morgun = tomorrow (fixed expression)

So while á often means on, you should treat á morgun as a set phrase meaning tomorrow, not translate it word-for-word each time.

Compare:

í gær = yesterday (literally in yesterday)
á morgun = tomorrow
These are just idiomatic time expressions.

What exactly does frídagur mean, and how is it formed?

Frídagur is a masculine noun meaning “day off” / “holiday (day off work or school)”.

It’s formed from:

frí = time off, leave, vacation, holiday
dagur = day

So frí + dagur → frídagur = a day on which you have time off.

Examples:

Það er frídagur á morgun. – There is a day off tomorrow.
Nemendur hafa frídag. – The students have a day off.

Related word:

helgidagur = religious/public holiday (holy day), e.g. Christmas Day, Easter Day.
A frídagur might be any day off, not necessarily religious.

Why is present tense used in það er frídagur á morgun if it’s about the future?

In Icelandic, the present tense is very often used to talk about scheduled or certain future events, especially with a time expression like á morgun (tomorrow).

So:

það er frídagur á morgun
literally: it is a day off tomorrow
naturally: there will be a day off tomorrow / it’s a holiday tomorrow.

This is similar to English in sentences like:

Tomorrow is Monday.
The train leaves at six tomorrow.

You could use a future-like form in Icelandic (það verður frídagur á morgun) but for fixed plans or calendar facts, the plain present is very normal.

Could I drop það and say “því er frídagur á morgun”?

No, that would not be natural in this meaning.

In því það er frídagur á morgun, the subordinate clause is:

það er frídagur á morgun – it is a holiday tomorrow.

Here það is a dummy subject (like it in it is raining). Icelandic normally needs that það in such sentences.

If you wanted to avoid það, you would typically change the word order of the clause, for example:

Ég sef lengur, því á morgun er frídagur.
(I sleep longer, because tomorrow is a day off.)

But simply því er frídagur á morgun sounds wrong in this sense.

Can the word order be changed, for example putting the reason clause first?

Yes, you can put the “because”-clause first, just like in English:

Því það er frídagur á morgun sef ég lengur.

This is grammatical, though in everyday speech many speakers might still prefer the original order. The main things to keep in mind:

The “because” clause stays together:
- því (að) það er frídagur á morgun – because it is a day off tomorrow
The main clause still follows normal Icelandic word order:
- sef ég lengur (verb typically in second position in its own clause, but conjunctions and subordinators can precede it).

How would I say “I will sleep in” more naturally in Icelandic? Is Ég sef lengur idiomatic?

Yes, Ég sef lengur is a perfectly natural and idiomatic way to express “I’ll sleep in” (in the sense of I’ll sleep longer than usual / I won’t get up early).

You might also hear:

Ég ætla að sofa lengur. – I’m going to sleep longer.
Ég ætla að sofa út. – I’m going to sleep in / sleep late (very idiomatic sleep in expression).

In your full sentence, Ég sef lengur, því það er frídagur á morgun, the meaning is exactly that:
“I’m going to sleep in because it’s a day off tomorrow.”

How do you pronounce Ég and því?

Approximate pronunciations (using English-like hints):

Ég:
- In standard pronunciation, something like “yeh” or “yei”, often with a soft g that is more like a voiced fricative (not a hard g as in go).
- Many speakers pronounce it closer to “yei”, kind of like the start of yay but shorter.
því:
- The þ is like the th in “thing” (unvoiced).
- The í is a long ee sound, like “see”.
- So því is roughly “thvee” (with a clear th
  - vee, long ee).

Exact phonetic values vary by dialect, but these approximations will make you understandable.