Kynlíf er hluti af lífinu.

Because the preposition af requires the dative case in Icelandic.

Here are the relevant forms of líf (life):

• líf = nominative/accusative, indefinite
• lífið = nominative/accusative, definite
• lífi = dative, indefinite
• lífinu = dative, definite

Since the sentence says af lífinu (of life / of the life), the noun after af must be in the dative, so lífinu is the correct form.

This is a very common learner question. Icelandic and English do not use the definite article in exactly the same way.

In English, we say part of life.
In Icelandic, the natural phrase is hluti af lífinu, literally part of the life.

That does not mean Icelandic is talking about one specific life in a narrow sense. It is just the idiomatic, natural way to express the idea. So it is best not to translate the article mechanically word-for-word here.

Because the sentence is making a general statement:

• Kynlíf er hluti af lífinu. = Sex is part of life.

It is saying that sex is a part of life, not the part of life.

If you said hlutinn, that would mean the part, which sounds much more specific and would usually need a clearer context.

Hluti is in the nominative singular.

That is because it is a predicate noun after the verb vera (to be). In Icelandic, nouns that follow vera and identify or classify the subject are usually in the nominative.

So in this sentence:

• Kynlíf = subject, nominative
• er = is
• hluti = predicate noun, also nominative
• af lífinu = prepositional phrase

They are:

• kynlíf = neuter
• hluti = masculine
• líf = neuter

This matters because Icelandic nouns change form depending on gender, number, definiteness, and case.

A useful extra point: in compounds, the last element usually determines the gender. Since kynlíf ends in -líf, and líf is neuter, kynlíf is neuter too.

Yes. Icelandic very often forms new words by making compounds.

Kynlíf is made from:

• kyn = sex, gender
• líf = life

Together, kynlíf means sex life / sexual life / sex depending on context. Icelandic prefers compounds in many places where English might use two words or a phrase.

Af often means of, from, or off, depending on context.

In this sentence, it is part of the expression:

• hluti af ... = part of ...

So:

• hluti af lífinu = part of life

This is a very useful pattern to remember:

• X er hluti af Y = X is part of Y

Er is the present tense, 3rd person singular form of vera (to be).

So:

• ég er = I am
• þú ert = you are
• hann/hún/það er = he/she/it is

The subject here, kynlíf, is a singular noun, so the correct verb form is er.

A rough learner-friendly pronunciation is:

• Kynlíf ≈ kin-leef
• er ≈ ehr
• hluti ≈ a voiceless-l sound + HLIU-ti or LU-ti depending on how carefully you say it
• af ≈ av
• lífinu ≈ LEE-vi-nu

So the whole sentence is roughly:

kin-leef ehr hlu-ti av LEE-vi-nu

A few important notes:

• Icelandic í is like ee in see
• f is often pronounced like v between vowels or voiced sounds, so lífinu sounds like leevinu
• hl is a special Icelandic sound: basically a voiceless l

Yes, but the version you have is the most neutral and natural one:

• Kynlíf er hluti af lífinu.

Icelandic word order is somewhat flexible, but changes usually affect emphasis, style, or information structure.

For example, another order might be possible in a special context, but for a basic statement, the original sentence is the best one to learn first.

Yes, you could, but it changes the nuance a bit.

• Kynlíf = sex in a general sense
• Kynlífið = the sex / sex life / sexuality, referring to it as a more specific or already-known concept

So:

• Kynlíf er hluti af lífinu. = a broad general statement
• Kynlífið er hluti af lífinu. = more like Sex is a part of life with a slightly more definite, discussion-based feel

Both can be grammatical, but the version without the article is a very natural general statement.

The structure is:

• Kynlíf = subject
• er = verb
• hluti = predicate noun
• af lífinu = prepositional phrase modifying hluti

So literally, it is:

• Sex is part of life

This is a useful model sentence for Icelandic:

[Noun] + er + [noun] + af + [dative noun]

For example:

• Þetta er hluti af verkefninu. = This is part of the project.