Þau fara þangað á hverjum föstudegi til að æfa sig.

Yes. Þau is a form of they.

More specifically, it is the neuter plural nominative form. Icelandic has grammatical gender, so they can appear as:

• þeir = masculine plural
• þær = feminine plural
• þau = neuter plural

Learners often see þau used for:

• a mixed-gender group
• a group of children
• a group where the gender is unknown or not important
• grammatically neuter plural nouns

In this sentence, þau is the subject, so it is in the nominative case.

Fara is the correct 3rd person plural present form of the verb að fara (to go).

A few present-tense forms are:

• ég fer = I go
• þú ferð = you go
• hann/hún/það fer = he/she/it goes
• við förum = we go
• þið farið = you all go
• þau fara = they go

So þau fara means they go.

This is a very common question.

• þar = there in the sense of being in a place
• þangað = there in the sense of going to a place

So:

• Þau eru þar. = They are there.
• Þau fara þangað. = They go there.

Because the sentence has fara (go), Icelandic uses þangað, which shows movement toward a place.

Á hverjum föstudegi is in the dative.

Here is the noun:

• föstudagur = Friday
• föstudegi = dative singular form

The preposition á can take different cases depending on meaning. In expressions of repeated time like on every Friday, Icelandic uses the dative:

• á hverjum föstudegi = every Friday / on every Friday

So this is a standard time expression, and that is why you see föstudegi, not föstudagur or föstudag.

Hverjum is the dative singular form of hver, which here means each or every.

It agrees with föstudegi in:

• case: dative
• number: singular
• gender: masculine

So:

• hverjum föstudegi = every Friday

Even though English uses every, Icelandic often uses a form of hver in this kind of expression.

Icelandic can express repeated time in the singular in this pattern:

• á hverjum föstudegi = every Friday

English often uses either:

• every Friday
• on Fridays

Icelandic could also use plural in some contexts, but á hverjum föstudegi is a very natural way to say every Friday.

So the singular is not strange here—it is just the normal structure with hverjum.

Til að introduces a purpose: in order to, to.

So:

• til að æfa sig = to practice / in order to practice

It tells you why they go there.

A slightly fuller form is:

• til þess að

But in everyday Icelandic, til að is very common and natural.

Because æfa sig is a reflexive expression.

• æfa can mean practice, train, rehearse
• sig means oneself/themselves in a reflexive sense

So æfa sig means something like:

• practice
• train
• work out
• literally, train themselves

This is different from practicing a direct object:

• æfa lagið = practice the song
• æfa íslensku = practice Icelandic

But when the idea is that the people are training or practicing themselves, Icelandic often uses æfa sig.

Sig is the reflexive pronoun used for the 3rd person.

It can refer back to:

• hann = himself
• hún = herself
• það = itself
• þeir/þær/þau = themselves

So even though English distinguishes himself / herself / itself / themselves, Icelandic uses sig for all of these in the appropriate case.

In this sentence, sig refers back to þau.

Yes, the time phrase could move.

The given order is very natural:

• Þau fara þangað á hverjum föstudegi til að æfa sig.

But Icelandic allows some flexibility, especially with adverbs and time expressions. For example:

• Þau fara á hverjum föstudegi þangað til að æfa sig.
• Á hverjum föstudegi fara þau þangað til að æfa sig.

The important thing to remember is that Icelandic main clauses usually follow the V2 rule: the finite verb tends to stay in the second position.

So if you move Á hverjum föstudegi to the front, the verb still comes next:

• Á hverjum föstudegi fara þau...

not

• Á hverjum föstudegi þau fara...

Yes, in a sentence like this, til að + infinitive clearly expresses purpose.

So the meaning is:

• they go there for the purpose of practicing
• they go there in order to practice

It answers the question why do they go there?

That is one of the most common uses of til að.

Because in Icelandic, days of the week are not capitalized unless they begin a sentence.

So:

• mánudagur = Monday
• þriðjudagur = Tuesday
• föstudagur = Friday

This is different from English, where weekday names are capitalized.

Yes, depending on context.

Æfa sig can mean:

• practice
• train
• exercise
• work out

If the context is sports or fitness, it may sound more like train/work out.
If the context is music, language, or performance, it may sound more like practice/rehearse.

So the exact English wording depends on what they are going there to do, but the Icelandic phrase itself is perfectly natural.