Learn Hardcore Icelandic: Þau tala íslensku heima.

þeir – “they” for a group that is all male (masculine plural)
þær – “they” for a group that is all female (feminine plural)
þau – “they” for a mixed group or a group of things / neutrals (neuter plural)

So Þau tala íslensku heima usually suggests either:

a mixed‑gender group of people, or
a group where gender is not being specified or is irrelevant.

How do you pronounce Þau?

Þ represents the unvoiced th sound as in think.

Approximate pronunciation of Þau:

Þ – like th in thing
au – a diphthong, roughly like English oi in boil, but with Icelandic flavour

So Þau sounds somewhat like “thoi”, but with a shorter, tighter vowel.

Why is the verb tala and not talar or some other form?

Tala is the present tense, 1st and 3rd person plural form of the verb að tala (to speak).

Present tense forms are:

ég tala – I speak
þú talar – you (singular) speak
hann / hún / það talar – he / she / it speaks
við tölum – we speak
þið talið – you (plural) speak
þeir / þær / þau tala – they speak

Since the subject is Þau (they), the correct form is tala.

Could you leave out Þau and just say Tala íslensku heima like in Spanish or Italian?

No. Icelandic is not a “null‑subject” language in the same way Spanish or Italian are.

You generally must include the subject pronoun:

Þau tala íslensku heima. – correct
*Tala íslensku heima. – sounds wrong or incomplete in normal speech

There are some special impersonal constructions where the subject is not expressed, but in a normal sentence like this one, you keep the pronoun.

Why is it íslensku and not íslenska here?

Íslenska is a feminine noun meaning “Icelandic (language).”
Its singular forms are roughly:

Nominative: íslenska
Accusative: íslensku
Dative: íslensku
Genitive: íslensku

After tala (to speak) with a language name as an object, Icelandic normally uses the accusative, so you say:

tala íslensku – speak Icelandic

Using íslenska here would be a nominative form and is not standard in this construction.

Is íslenska / íslensku a noun or an adjective here?

In this sentence, íslensku is a noun meaning “Icelandic (the language).”

There is also an adjective íslenskur, íslensk, íslenskt meaning “Icelandic” (e.g. Icelandic man, Icelandic food), but that is not what is used here.

So:

íslenska (íslensku) – the language (noun)
íslenskur / íslensk / íslenskt – Icelandic as an adjective

Why is there no word for “the” or “a” before íslensku?

Language names in Icelandic are normally used without any article in this kind of construction.

You say:

Þau tala íslensku. – They speak Icelandic.

You would not normally say something like *tala tungumálið íslensku (speak the Icelandic language) unless you have a very specific, unusual context. In everyday speech, the bare language name is used.

What is the difference between tala íslensku and tala á íslensku?

Both are common, but there is a nuance:

tala íslensku – literally “speak Icelandic”
- Focus on knowing/using the language
- Very common when talking about what languages someone speaks
tala á íslensku – literally “speak in Icelandic”
- Slightly more focus on the medium of communication in a given situation
- Common when contrasting languages or talking about how something is said

In many everyday situations, they are close in meaning and both acceptable:

Þau tala íslensku heima.
Þau tala á íslensku heima.

Both can be heard, though tala íslensku is very standard.

What does heima literally mean, and why is there no preposition like “at”?

Heima is an adverb meaning “at home.”

Because it is an adverb, it already contains the “at” meaning inside it, so you do not add a preposition:

Ég er heima. – I am at home.
Þau tala íslensku heima. – They speak Icelandic at home.

You do not say *á heima in standard Icelandic.

What is the difference between heima and heim?

They express location vs direction:

heima – “at home” (location, where?)
- Ég er heima. – I am at home.
- Þau tala íslensku heima. – They speak Icelandic at home.
heim – “home” (direction, to where?)
- Ég fer heim. – I am going home.
- Hann keyrir heim. – He drives home.

So in this sentence, you need heima, because it describes where they are when they speak.

Can the word order change, for example Þau tala heima íslensku or Heima tala þau íslensku?

Icelandic word order is relatively flexible, but not all orders sound equally natural.

Neutral, most natural order here:

Þau tala íslensku heima.

Other possibilities:

Heima tala þau íslensku. – Also possible; begins with the place for emphasis: “At home, they speak Icelandic.”

But:

*Þau tala heima íslensku. – Feels wrong/awkward; heima normally comes at the end or near the beginning, not between verb and object here.

Safe rule for learners:

Keep subject – verb – object – place as in the original sentence, unless you have learned specifically how to front adverbs for emphasis.

How would you say “They don’t speak Icelandic at home” and “They only speak Icelandic at home”?

Negation:

Þau tala ekki íslensku heima.
- ekki is the normal word for “not.”
- It usually comes after the finite verb (tala) and before the object (íslensku).

“Only” (in the sense of “exclusively”):

Þau tala bara íslensku heima.
Þau tala eingöngu íslensku heima. (more formal / stronger)

Word order stays similar; you simply insert ekki, bara, or eingöngu after tala and before íslensku.