Learn Hardcore Icelandic: Hún vill alltaf hafa með sér góðan ferðafélaga þegar hún ferðast langt. - She always wants to have a good travel companion with her when she travels far.

Hún hefur góðan ferðafélaga = She has a good travel companion. (simple possession, present)
Hún vill hafa góðan ferðafélaga = She wants to have a good travel companion. (desire)

So the pattern is:

vilja (to want) + infinitive
- Ég vil fara = I want to go
- Hún vill hafa = She wants to have

What exactly does hafa með sér mean? How is it different from just hafa or taka með sér?

Hafa með sér literally means have with oneself, i.e. have something/someone along, on you, accompanying you.

hafa alone = to have, possess
- Hún hefur peninga = She has money.
hafa með sér = to have with you (carry/bring and have)
- Hún vill alltaf hafa með sér góða vini = She always wants to have good friends with her.

Taka með sér means to take/bring along (the act of bringing):

Ég tek regnhlíf með mér = I take/bring an umbrella with me.
After you have taken it, you hafa hana með þér (have it with you).

In this sentence, hafa með sér góðan ferðafélaga focuses on the state of being accompanied by a good travel companion, not the moment of bringing them.

Why is it með sér and not með henni for with her? What does sér do?

Sér is the third-person reflexive pronoun in the dative case. It refers back to the subject of the clause (here hún).

með sér = with herself / with himself / with themselves
með henni = with her (someone else, not the subject)

So:

Hún vill hafa bókina með sér = She wants to have the book with her (on her person).
Hún vill hafa bókina með henni = She wants to have the book with her (another woman), i.e. the book should be with that other woman.

In your sentence, með sér clearly says the travel companion is with her, the subject.

Why is it góðan ferðafélaga and not góður ferðafélagi?

Because góðan ferðafélaga is in the accusative singular masculine, and the adjective must agree in case, number, and gender with the noun.

Nominative: góður ferðafélagi = a good travel companion (as subject)
- Góður ferðafélagi er mikilvægur. = A good travel companion is important.
Accusative: góðan ferðafélaga = a good travel companion (as direct object)
- Hún vill hafa góðan ferðafélaga. = She wants to have a good travel companion.

Here, ferðafélaga is the direct object of hafa, so it must be in the accusative. The adjective changes to góðan (accusative masculine singular, strong declension).

Why does ferðafélagi become ferðafélaga in this sentence?

Ferðafélagi is a weak masculine noun. Many weak masculine nouns have:

-i in the nominative singular
-a in the accusative, dative, and genitive singular

So the singular forms are:

Nominative: ferðafélagi (companion as subject)
Accusative: ferðafélaga (as object)
Dative: ferðafélaga
Genitive: ferðafélaga

In góðan ferðafélaga, it is accusative singular (direct object of hafa), so you see -a.

Why is the adjective góðan masculine when hún (she) is feminine?

Adjectives in Icelandic agree with the noun they modify, not with the subject’s natural gender.

ferðafélagi is grammatically masculine (a masculine noun)
Therefore the adjective must be masculine: góður / góðan ferðafélaga

Even though the person is female (hún), the word for companion in this form is masculine, so the adjective follows the noun’s grammatical gender, not the person’s sex.

What exactly does ferðafélagi mean? Is it different from vinur (friend)?

Ferðafélagi is a compound:

ferð = trip, journey
félagi = companion, mate, member

So ferðafélagi literally means travel companion / travel mate.

It doesn’t automatically imply close friendship, just that this is the person you travel with. Vinur means friend in general. A ferðafélagi can be a friend, partner, colleague, etc., but the word focuses on the role in travel, not the personal relationship.

What is the role of alltaf here, and could it go in a different place?

Alltaf means always and here it is an adverb modifying the verb phrase vill hafa með sér. In Icelandic main clauses, the finite verb usually comes second, and adverbs like alltaf often come right after it or after the subject+verb.

Your sentence has:

Hún vill alltaf hafa með sér ...

Other natural options:

Hún alltaf vill hafa með sér góðan ferðafélaga ... (possible but less neutral; sounds marked/emphatic)
Hún vill hafa með sér góðan ferðafélaga alltaf þegar hún ferðast langt. (= She wants to have a good travel companion with her always when she travels far.)

The given word order, with alltaf after vill, is the most common and neutral.

Why is hún repeated in þegar hún ferðast langt? Could I just say þegar ferðast langt?

In Icelandic, you normally repeat the subject in a new finite clause, even if it is the same person.

Hún vill ... þegar hún ferðast langt.

Leaving out hún would be ungrammatical here:

✗ *þegar ferðast langt – incorrect

You can sometimes omit the subject in very short, informal coordinated clauses when it’s clearly the same, but after þegar (a subordinator), you normally must use an explicit subject: þegar hún ferðast.

Why is ferðast used instead of fara? Don’t both mean “to go/travel”?

Both are related, but they have different nuances:

fara = to go, leave, move from one place to another (often a single trip or movement)
- Hún fer til Parísar. = She goes to Paris.
ferðast = to travel (as an activity, to be travelling, to do trips)
- Hún ferðast mikið. = She travels a lot.

Ferðast is formed with the -st ending and often has a more middle/reflexive or habitual/ongoing feel: travelling as an activity, sometimes over longer distances or as a general habit.

So þegar hún ferðast langt means when she travels far / when she goes on long journeys, not just a single act of going somewhere once.

What is the function of the -st in ferðast? Is it reflexive?

The -st ending marks a middle voice (miðmynd) in Icelandic. It can have several functions:

Reflexive meaning (do something to oneself)
Passive-like meaning
Or just form a separate verb with its own meaning

Ferðast comes from fara + -st, but in modern Icelandic it is just understood as to travel, not as a transparent reflexive form. You don’t say fara sig; you just use ferðast.

So you can treat ferðast as an ordinary verb meaning to travel, whose 3rd person singular present is also ferðast.

Why is langt used, and what kind of word is it here? Could you say lengi instead?

Langt here is an adverb, meaning far. It is actually the neuter singular accusative form of the adjective langur (long), used adverbially. This is a common pattern in Icelandic: neuter singular forms of adjectives used as adverbs.

Hún ferðast langt. = She travels far.
Hann hljóp hratt. = He ran fast. (neuter of hraður)

Lengi means for a long time, focusing on duration, not distance:

Hún ferðast lengi. = She travels for a long time.

So:

ferðast langt = travel a long way (distance)
ferðast lengi = travel for a long time (time)

In your sentence, langt correctly expresses far / long-distance.

Why is the present tense ferðast used when this seems to describe a general habit, not something happening right now?

Icelandic uses the present tense very broadly:

For actions happening now
For general habits
For timeless truths
Often where English might use “when she travels” or even “when she is travelling”

So:

Þegar hún ferðast langt can mean when she travels far / whenever she travels far.

You don’t need a special habitual form; the simple present does that job in Icelandic.

Could you rephrase the sentence in a simpler way and still keep the same meaning?

Yes, for example:

Hún vill alltaf hafa góðan ferðafélaga með sér þegar hún fer í langa ferð.
- Literally: She always wants to have a good travel companion with her when she goes on a long trip.

Here:

ferðast langt is rephrased as fer í langa ferð (goes on a long trip),
but the overall meaning is essentially the same.