Learn Hardcore Icelandic: Ferðafélagarnir tala aðeins íslensku á ferðalögum sínum til að æfa sig sérstaklega vel. - The travel companions speak only Icelandic on their trips in order to practice especially well.

segja = to say / to tell (it needs some content: say something, tell someone something)
- Hann segir satt. – He tells the truth.
mæla is more old-fashioned/literary and not the usual everyday verb for speaking a language.

So tala is the right choice when you’re talking about what language someone speaks.

What case is íslensku in, and why that case?

Íslensku is in the accusative singular.

The noun is íslenska (Icelandic, the language), a weak feminine noun:

nominative: íslenska
accusative: íslensku
dative: íslensku
genitive: íslensku

The form -u here is accusative (and also dative/genitive, but the syntax tells us which).

The verb tala (to speak) usually takes the accusative for the language:

að tala íslensku – to speak Icelandic
að tala ensku – to speak English

So íslensku is the direct object of tala in the accusative.

Could you also say tala á íslensku, and is it different from tala íslensku?

Yes, tala á íslensku is also possible, but there is a nuance.

tala íslensku
Very normal and common; simply “speak Icelandic” as a language.
tala á íslensku
Literally “speak in Icelandic”. This often emphasizes the medium of communication (the language used between people), and is very natural in sentences like:
- Við tölum saman á íslensku. – We speak together in Icelandic.

In your sentence, tala aðeins íslensku is perfectly standard and probably the most textbook-like formulation.

What does á ferðalögum sínum literally mean, and why is ferðalögum in that form?

Á ferðalögum sínum literally means “on their trips/journeys” or “when they are travelling.”

ferðalag = trip, journey (neuter noun)

Its relevant forms:

nom./acc. plural: ferðalög = trips
dative plural: ferðalögum = on/at/in trips

The preposition á can take dative when it indicates location/state (where something happens), and accusative when it indicates movement into/onto something.

Here it’s location (“on their trips”), so á + dative → á ferðalögum.

Then we add the reflexive possessive:

sínum = “their (own)” in dative plural, agreeing with ferðalögum.

So:

á (on)
ferðalögum (trips, dative plural)
sínum (their own, dative plural)

→ á ferðalögum sínum = on their (own) trips.

Why is á used here and not í? Could you say í ferðalögum sínum?

The idiomatic choice here is á ferðalögum sínum.

á is commonly used with:
- time periods or occasions (á veturna – in the winters)
- activities/events/trips (á ferðalögum – on trips, á tónleikum – at concerts)
í ferðalögum sínum would sound odd or unnatural in this context. Í is more “in/inside” (in a house, in a car, in a box) or in more abstract “in a situation” uses.

So for “on their travels/on trips”, Icelandic prefers á ferðalögum (sínum).

How does sínum work here, and what’s the difference between sínum and þeirra?

Sínum is the reflexive possessive pronoun, corresponding to “their own” here.

sinn, sín, sitt = his/her/its/their own (reflexive, refers back to the subject)
sínum is the dative plural form, agreeing with ferðalögum (neuter, dative plural).

Ferðafélagarnir … á ferðalögum sínum
= The travel companions … on their own trips.

If you used þeirra instead:

á ferðalögum þeirra
This normally means “on their trips” where “their” refers to some other group, not the subject.

So:

sínum = their own (same people as the subject)
þeirra = their (some other group, or at least not necessarily the subject)

That’s why sínum is used here.

What is the function of til að in til að æfa sig?

Til að introduces a purpose clause, roughly “in order to” in English.

til = to / for (often “for the purpose of”)
að æfa = to practise (infinitive)

So:

til að æfa sig = “in order to practise” / “so as to practise.”

You might also see the slightly longer til þess að æfa sig, which is also correct. In many everyday sentences, til að is enough and very common.

Why is it æfa sig and not just æfa, or æfa íslensku?

Æfa sig is a very common pattern in Icelandic and literally means “practise oneself”, i.e. practise / train (one).

æfa sig = to practise (oneself), to train, to get practice

In this sentence:

til að æfa sig = to get practice, to practise (themselves).

Alternatives:

æfa íslensku = practise Icelandic (the language specifically)
æfa sig í íslensku = practise (oneself) in Icelandic

All are possible in the right context. Your sentence chooses æfa sig, focusing on the speakers’ practice/training rather than explicitly repeating “Icelandic” again.

What does sig refer to here, and why is it reflexive?

Sig is the reflexive pronoun, referring back to the subject of the sentence.

Subject: Ferðafélagarnir (the travel companions)
Verb phrase: til að æfa sig (in order to practise themselves)

So sig = “themselves” (i.e. the travel companions).

It’s reflexive because the people who are doing the practising are also the ones being practised/trained. Icelandic uses sig for this “the subject does something to/for itself” meaning.

What does sérstaklega vel add, and could you just say vel?

vel = well
sérstaklega = especially, particularly

Together:

sérstaklega vel = especially well / particularly well / very well

You could say just til að æfa sig vel (“to practise well”), which is correct and natural.

Adding sérstaklega makes it stronger: they don’t just want to practise well; they want to practise especially well, putting extra emphasis on quality or intensity of the practice.

Is the word order fixed, or could elements like aðeins or á ferðalögum sínum move?

Icelandic allows some flexibility, though there are preferred patterns.

Current order:

Ferðafélagarnir (subject)
tala (verb – 2nd position)
aðeins (adverb “only”)
íslensku (object)
á ferðalögum sínum (adverbial phrase)
til að æfa sig sérstaklega vel (purpose clause)

You’ll normally keep:

the finite verb in second position in main clauses
the purpose clause (til að …) toward the end

You can move some adverbials for emphasis, for example:

Ferðafélagarnir tala á ferðalögum sínum aðeins íslensku…
(puts more emphasis on on their trips)
Ferðafélagarnir tala íslensku aðeins á ferðalögum sínum…
(suggests they only do it on their trips, not necessarily at other times)

All of these are grammatical; the differences are mostly about emphasis and focus. The original order is very natural and neutral.