La temperamento de la katido estas trankvila.

Here de shows a relationship like English of or 's.

So:

• la temperamento de la katido = the temperament of the kitten
• In natural English, that is often simply the kitten's temperament

Esperanto often uses de where English uses a possessive form.

Because each definite noun phrase gets its own la.

• la temperamento = the temperament
• la katido = the kitten

So La temperamento de la katido literally has the before both nouns, just as English can say the temperament of the kitten.

If you removed the second la, de katido would sound less definite, more like of a kitten or kitten-related depending on context.

Katido means kitten.

It is built from:

• kat- = cat
• -id- = offspring / young one / descendant
• -o = noun ending

So katido is literally a young cat.

This is a very common Esperanto word-building pattern.

Because -a is the normal ending for an adjective in Esperanto.

Here trankvila means calm and describes temperamento.

A very important point for English speakers: in Esperanto, -a does not mean feminine. It simply means adjective.

So:

• trankvila = calm
• bona = good
• granda = big

Because trankvila is an adjective, and trankvile would be an adverb.

In this sentence, calm describes the noun temperamento, so you need an adjective:

• La temperamento ... estas trankvila. = The temperament is calm.

If you used trankvile, it would mean calmly, which describes how something happens, not what something is like.

Compare:

• Li parolas trankvile. = He speaks calmly.
• Li estas trankvila. = He is calm.

It describes temperamento, not katido.

The structure is:

• La temperamento = the subject
• de la katido = a modifying phrase telling whose temperament it is
• estas trankvila = is calm

So the sentence says that the temperament is calm.

That is why trankvila is singular: it matches temperamento.

Because there is no direct object in this sentence.

This is a sentence with esti (to be), which links the subject to a description:

• La temperamento = subject
• estas = is
• trankvila = predicate adjective

With esti, the descriptive adjective normally does not take -n.

Also, de la katido is a prepositional phrase, and the noun after de normally does not take -n either.

So no word here needs the accusative ending.

Yes. That is a perfectly good alternative.

It means essentially the same thing:

• La temperamento de la katido estas trankvila.
• La katido havas trankvilan temperamenton.

The second version uses havas (has), so now temperamenton is a direct object and takes -n.
Its adjective also agrees with it:

• trankvilan temperamenton

This is a useful comparison because it shows when the accusative appears.

Not completely, but the version you have is the most neutral and easiest for learners.

The normal order here is:

• subject: La temperamento de la katido
• verb: estas
• description: trankvila

You could change the order for emphasis, for example:

• Trankvila estas la temperamento de la katido.

But that sounds more marked or literary. For ordinary use, the original order is best.

Because Esperanto uses many international roots, especially for ideas that already have widely shared forms across European languages.

So temperamento is easy to recognize if you know English or Romance-language vocabulary.

Esperanto then adds its regular endings:

• temperament-o = temperament
• trankvil-a = calm
• kat-id-o = kitten

This mix of familiar roots and very regular endings is one of the main features of Esperanto grammar and vocabulary.