Ŝi tre ŝatas junan poetinon, kies poemoj sonas simple, sed restas en la kapo.

Because junan poetinon is the direct object of ŝatas. In Esperanto, the direct object normally takes the accusative ending -n.

• ŝi ŝatas poetinon = she likes a poet
• juna poetino = a young poet
• junan poetinon = a young poet, as the direct object

The adjective must agree with the noun, so both words take -n:

• juna → junan
• poetino → poetinon

-in- is the feminine suffix. It marks someone or something as female.

• poeto = poet
• poetino = female poet

So poetinon is the accusative form of poetino.

Esperanto has la for the, but it does not have an indefinite article like English a/an.

So:

• junan poetinon = a young poet
• la junan poetinon = the young poet

No extra word is needed for a.

tre is an adverb, and here it modifies the verb ŝatas. Esperanto often puts adverbs before the word they modify.

So tre ŝatas means likes very much or really likes.

You may also hear other patterns in Esperanto, but tre ŝatas is perfectly normal and easy to understand.

kies means whose. It introduces a relative clause that shows possession.

In this sentence, kies refers back to poetinon:

• poetinon, kies poemoj... = the poet, whose poems...

So the clause is describing the poet by talking about her poems.

Because kies is what links the relative clause directly to poetinon.

• kies poemoj = whose poems
• ŝiaj poemoj = her poems

If you said ŝiaj poemoj, it would still be understandable in context, but kies is the normal way to say whose in a relative clause.

No. kies is invariable. It does not take -j or -n.

So you get forms like:

• kies poemo = whose poem
• kies poemoj = whose poems
• kies poemon = whose poem, as object

The noun after kies shows number and case if needed, not kies itself.

Because poemoj is the subject of sonas and restas inside the relative clause.

• kies poemoj sonas simple
• kies poemoj restas en la kapo

Subjects do not take -n. Only direct objects do.

The -j shows plural:

• poemo = poem
• poemoj = poems

This is a point that often feels strange to English speakers.

English says the poems sound simple, using an adjective. In Esperanto, with verbs like soni, people often use an adverb to describe the way something comes across:

• sonas simple = sounds simple / sounds in a simple way

So simple here is the adverb form, not the adjective.

For a learner, this is a good and safe pattern:

• aspektas bele = looks beautiful
• sonas strange = sounds strange
• odoras bone = smells good

An adjective after such verbs can appear in some contexts, but sonas simple is the straightforward choice here.

Because Esperanto verbs do not agree with the subject in number or person.

In the present tense, the verb always ends in -as:

• mi restas
• ŝi restas
• ili restas
• la poemoj restas

So even with a plural subject like poemoj, the verb is still restas.

Literally, it means in the head. Idiomatically, it means something like:

• stays in your head
• sticks in your mind
• is memorable

So restas en la kapo suggests that the poems linger in the memory.

Esperanto often uses la with body parts and similar expressions when the reference is clear or general.

So en la kapo can mean something like in one’s head / in the mind without naming whose head it is.

Also, the singular is natural here because it is an idiomatic expression. It is not literally talking about several physical heads; it is talking about the mental effect of the poems.

Because kies poemoj sonas simple, sed restas en la kapo is a relative clause describing poetinon.

In standard Esperanto writing, a relative clause is usually set off with a comma:

• poetinon, kies poemoj...

The comma helps show where the main clause ends and the descriptive clause begins.