Ef hún sleppir hádegislúrnum, verður kvöldið erfiðara fyrir okkur bæði.

Because hún is third-person singular, and sleppir is the third-person singular present form of sleppa.

A few present-tense forms are:

• ég sleppi = I skip / let go
• þú sleppir = you skip / let go
• hún sleppir = she skips / lets go

So Ef hún sleppir ... means If she skips ...

Here, sleppa means to skip or to go without.

This verb has several related meanings in Icelandic, such as:

• let go / release
• escape / get away
• skip / omit

In this sentence, the meaning is clearly the skip sense:

• sleppa hádegislúrnum = skip the nap

Because sleppa in this meaning takes the dative case, and hádegislúrnum is the dative singular definite form.

Breakdown:

• hádegislúr = midday nap / afternoon nap
• hádegislúrinn = the nap
• hádegislúrnum = the nap (dative)

So:

• hún sleppir hádegislúrnum = she skips the nap

This is one of those verb-case pairings you simply have to learn:
sleppa + dative

In the common meanings relevant to learners, sleppa very often takes the dative.

For example:

• sleppa þessu = skip this / let this go
• sleppa fundinum = skip the meeting
• sleppa boltanum = let go of the ball

So for this sentence, the important thing to remember is:

• sleppa einhverju = skip something / let go of something

Because Icelandic commonly uses the present tense in if-clauses when talking about a future possibility.

So this works just like English:

• If she skips the nap, the evening will be harder...

Even though it refers to the future, Icelandic uses:

• sleppir = present
• verður = present form, but with future meaning in context

This is very normal Icelandic usage.

Because verða means become or get, and here the idea is that the evening will become harder as a result.

So:

• kvöldið verður erfiðara = the evening will be harder

Using er would mean simply is, which would not express the same change/result as clearly.

In Icelandic, verða is very common when English would use will be in a result like this.

Because Icelandic is a verb-second language.

When the sentence begins with the subordinate clause Ef hún sleppir hádegislúrnum, that whole clause takes the first position. Then the finite verb of the main clause comes next:

• Ef hún sleppir hádegislúrnum, verður kvöldið erfiðara ...

So the pattern is:

• [subordinate clause] + finite verb
  • subject

This is very typical Icelandic word order.

Because the sentence means harder, not just hard.

• erfitt = difficult / hard
• erfiðara = more difficult / harder

The comparative is used because the idea is:

• if she skips the nap, the evening will be harder than it otherwise would be

So erfiðara is the natural choice.

Because kvöldið is a neuter singular noun, and the adjective has to match it.

• kvöld = evening, a neuter noun
• kvöldið = the evening

The comparative adjective agrees with the noun:

• masculine/feminine forms can be erfiðari
• neuter singular is erfiðara

So:

• kvöldið verður erfiðara = correct

Because Icelandic often uses the definite form when talking about a specific, understood situation.

Here, it means the evening in the context already being discussed — the evening ahead, the one affected by whether she naps or not.

So:

• kvöld = evening
• kvöldið = the evening

Using kvöldið makes it sound like a specific evening, which is exactly what this sentence refers to.

It means for both of us or for us both.

Breakdown:

• fyrir = for
• okkur = us
• bæði = both

So the whole phrase means that the evening will be harder for the two of us, not just for one person.

Because bæði is the form used for both in this accusative-plural context, especially when referring to a mixed pair or when gender is not being specified in the phrase.

In practice, learners should recognize:

• við bæði = we both
• okkur bæði = us both

So fyrir okkur bæði is a very natural way to say for both of us.

Because here the phrase is governed by fyrir in its accusative use, not the dative use.

A tricky point is that okkur looks the same in both accusative and dative, so the case difference is not visible on the pronoun itself. But it shows up in bæði.

So in this sentence:

• fyrir okkur bæði = for both of us

If the structure required dative, you would expect a dative form like báðum, but that is not what is happening here.

Yes. Icelandic very often forms nouns by combining smaller words.

Here:

• hádegi = midday / noon
• slúr = nap / snooze

So:

• hádegislúr = midday nap / afternoon nap

Then the sentence uses the definite dative form:

• hádegislúrnum = the nap (in the form required by the verb)

It is presented as a real, possible condition: if this happens, then that result follows.

That is why Icelandic uses ordinary indicative forms:

• Ef hún sleppir ...
• verður ...

This is similar to English:

• If she skips the nap, the evening will be harder...

It is not framed as a remote or impossible hypothetical.