Learn Hardcore Icelandic: Vonandi slekk ég á símanum til þess að sofa betur. - Hopefully I turn off the phone in order to sleep better.

Putting vonandi first slightly emphasizes the hope itself; putting it later feels a bit more neutral. Grammatically, both are fine as long as the finite verb stays in second position overall (see the word-order question below).

Why is it slekk and not slökkva?

Slökkva is the infinitive form of the verb (to turn off, to extinguish).

In the sentence you need a finite, present-tense, 1st person singular form:

ég slekk – I turn off

So:

að slökkva – (to) turn off
ég slekk – I turn off

This verb is a bit irregular: the stem vowel changes (ö → e) and the ending shortens, giving slekk instead of something like slökkvi. You don’t need to worry about the full paradigm at this stage; just learn að slökkva – ég slekk as a pair.

Why do you say slekk á símanum instead of just using the phone as a direct object?

In Icelandic, slökkva á e-u (with á + dative) is the normal way to say turn something off (a light, phone, TV, etc.).

So you don’t usually say:

✗ slekk símann

Instead you say:

✓ slekk á símanum – literally I turn off on the phone

Other common phrases with the same pattern:

slökkva á ljósinu – turn off the light
slökkva á sjónvarpinu – turn off the TV

Think of slökkva á as a fixed combination: verb + preposition, and the thing being turned off is in the dative case.

Why is it símanum and not sími, síma, or símann?

Símanum is the definite dative singular of sími (phone).

The basic forms (singular) are:

Nominative: sími – (a/the) phone
Accusative: síma
Dative (indefinite): síma
Dative (definite): símanum – the phone
Accusative (definite): símann – the phone (as direct object)

Here, the preposition á and the verb pattern slökkva á e-u require the dative case, and you’re talking about the phone, so you need:

á símanum – on the phone (dative + definite article)

So the combination of:

verb pattern: slökkva á e-u
required case: dative
definiteness: the phone

gives you á símanum.

Can I use a different form like símann or just síma here, and what would it mean?

Not with slökkva á, because that pattern specifically wants á + dative.

However, in other contexts you might see:

Ég á símann. – I own the phone. (accusative definite, direct object)
Ég tala í síma. – I talk on a phone / on the phone. (dative, indefinite)
Ég tala í símanum. – I talk on the phone. (dative, definite)

In Vonandi slekk ég á símanum, changing símanum to símann or síma would simply be ungrammatical with slökkva á.

What does til þess að literally mean, and why is it used here?

Til þess að is a very common way to express purpose, like in order to / so as to.

Literally:

til – to, toward, for (and it governs the genitive case)
þess – genitive of það (of that/it)
að sofa betur – to sleep better

So til þess að sofa betur is something like:

for that (purpose), to sleep better
idiomatically: in order to sleep better

You use it here to explain the purpose of turning off the phone.

Could I say til að sofa betur instead of til þess að sofa betur?

Yes. Both are grammatically correct and common:

til þess að sofa betur
til að sofa betur

The differences:

til þess að is a bit more explicit and slightly more formal, especially in writing.
til að is a bit shorter and more colloquial, but fully acceptable.

For everyday speech, til að sofa betur is perfectly fine. For careful or formal writing, many people prefer til þess að.

Why is it að sofa (infinitive) and not something like að sofi?

In Icelandic, when að introduces an infinitive clause of purpose like this, it is followed by the infinitive form of the verb:

að sofa – (to) sleep
að borða – (to) eat
að læra – (to) study

So:

til þess að sofa betur – in order to sleep better

Forms like sofi are finite (here: a present subjunctive form), which you would see after að when it works as a subordinator (that) introducing a full clause with subject, e.g.:

Ég vona að ég sofi betur. – I hope that I sleep better.

In your sentence, you have a purpose construction, so infinitive (sofa) is the correct choice.

What is betur, and how is it related to vel?

Vel means well (adverb).
Betur is its comparative form: better.

Adverb paradigm:

vel – well
betur – better
best – best

So:

sofa vel – to sleep well
sofa betur – to sleep better
sofa best – to sleep best

In English you’d say to sleep better, so betur is the natural choice here.

Why is the word order Vonandi slekk ég á símanum and not Ég slekk vonandi á símanum?

Icelandic has a verb-second (V2) word order in main clauses:

Some element (subject, adverb, time phrase, etc.)
Finite verb
Subject (if it wasn’t in position 1)
The rest of the sentence

In your sentence:

1st position: Vonandi
2nd position (finite verb): slekk
Then subject: ég
Then the rest: á símanum til þess að sofa betur

So Vonandi slekk ég… obeys V2.

You can also say:

Ég slekk vonandi á símanum…

Here:

1st: Ég
2nd: slekk
then adverb: vonandi

Both are correct; they just differ in what you put in first position and thus in emphasis. Starting with vonandi highlights the hope more strongly.

Could I say something like Ég vona að ég slekk á símanum til þess að sofa betur instead of using vonandi?

Yes, that’s another perfectly good way to express a similar idea:

Ég vona að ég slekk á símanum til þess að sofa betur.
– I hope that I turn off the phone in order to sleep better.

Differences:

vonandi is a sentence adverb:
- Vonandi slekk ég á símanum… – Hopefully I turn off the phone…
  It doesn’t say explicitly who is hoping; it just presents the hope as a general attitude.
Ég vona að… is a full verb phrase with a clear subject (ég):
- literally I hope that…

Both are common; vonandi is shorter and very frequent in spoken Icelandic.

How are þ and ð pronounced here, especially in þess and að?

In this sentence you see þ in þess and ð in að.

Pronunciation (approximate English equivalents):

þ (thorn), as in þess
- voiceless th, like th in thing, thought.
ð (eth), as in að
- voiced th, like th in this, that.

So:

þess ≈ thess (with th as in thing)
að ≈ a(th) (short a plus th as in this; the final ð often weakens a bit in fast speech)

Getting the þ / ð contrast right is one of the characteristic features of Icelandic pronunciation.