Þau tala um kynlífið sitt.

Þau means they.

It is the neuter plural form of the 3rd-person pronoun. In real usage, þau is often used for:

• a mixed-gender pair or group
• a pair being treated together as a unit
• sometimes people whose gender is not the main point

So in this sentence, Þau tala... = They talk...

Because the subject is þau, which is 3rd person plural.

The present tense of tala (to talk / speak) changes by person and number:

• ég tala = I talk
• þú talar = you talk
• hann/hún/það talar = he/she/it talks
• við tölum = we talk
• þið talið = you all talk
• þeir/þær/þau tala = they talk

So Þau tala is the correct form for they talk.

Um means about in this sentence.

So tala um is the normal way to say talk about:

• tala um veðrið = talk about the weather
• tala um vinnuna = talk about work
• tala um kynlífið sitt = talk about their sex life

Um takes the accusative.

That means the noun phrase after it has to be in the accusative case:

• um kynlífið sitt

In this particular sentence, the forms do not visibly change much, because:

• kynlíf is a neuter singular noun
• many neuter singular nominative and accusative forms look the same
• sitt also has the same form in nominative and accusative neuter singular

So the case is accusative, even though the form does not obviously look different.

Because kynlífið is the definite form: the sex life / the sex.

In Icelandic, the definite article is usually attached to the end of the noun:

• kynlíf = sex life / sex
• kynlífið = the sex life / the sex

With possessives, Icelandic very often uses this pattern:

noun + definite article + possessive

So:

• kynlífið sitt = their own sex life

Even though English just says their sex life without the, Icelandic normally uses the definite noun here.

Because sitt is a reflexive possessive. It refers back to the subject of the sentence.

So in:

• Þau tala um kynlífið sitt

sitt means their own.

This tells us that the sex life belongs to the same people as the subject þau.

If you used þeirra instead, it would usually mean their in a non-reflexive sense — in other words, someone else’s:

• Þau tala um kynlífið þeirra
  = They talk about their sex life
  but usually understood as someone else’s sex life, not their own.

So if the meaning is their own, sitt is the right choice.

Because sinn / sína / sitt agrees with the thing possessed, not with the possessor.

Here, the possessed noun is kynlífið, which is:

• neuter
• singular
• accusative

So the matching form is sitt.

Compare:

• bílinn sinn = his/her/their own car
  (bílinn is masculine singular accusative)
• bókina sína = his/her/their own book
  (bókina is feminine singular accusative)
• kynlífið sitt = his/her/their own sex life
  (kynlífið is neuter singular accusative)

This is a very important Icelandic pattern.

Because that is the most normal Icelandic word order for possessives in everyday language:

• bókin mín = my book
• húsið okkar = our house
• kynlífið sitt = their own sex life

So Icelandic often prefers:

• noun + possessive

rather than the English-style order:

• possessive + noun

A fronted possessive is sometimes possible, but it usually sounds more emphatic, contrastive, or stylistically marked.

Yes. Icelandic very often makes compound nouns where English uses two words.

So:

• English: sex life
• Icelandic: kynlíf

It is built from:

• kyn = sex / gender
• líf = life

Putting them together into one word is completely normal in Icelandic.

The letter þ is pronounced like th in thing, never like th in this.

So Þau begins with that voiceless th sound.

The au is a diphthong. A rough English approximation is something like thoy, though the Icelandic vowel quality is not exactly the same as English.

So a useful learner approximation is:

Þau ≈ thoy

That will get you close enough to be understood.