La kaverno ŝajnas stranga.

La means the. It is the definite article in Esperanto.

So:

• la kaverno = the cave
• kaverno = a cave / cave depending on context

Esperanto has only one definite article, la, and it does not change for gender, number, or case.

In Esperanto, nouns normally end in -o.

So kaverno is a noun meaning cave or cavern.

This is one of the most basic patterns in Esperanto:

• -o = noun
• -a = adjective
• -e = adverb
• -i = infinitive verb

Because kaverno is the thing being talked about, it takes the noun ending -o.

Because stranga is an adjective, and adjectives in Esperanto end in -a.

Here it describes kaverno, so:

• kaverno = cave
• stranga = strange

Together, the sentence says that the cave seems strange.

Because ŝajni means to seem, while esti means to be.

So there is an important difference:

• La kaverno estas stranga. = The cave is strange.
• La kaverno ŝajnas stranga. = The cave seems strange.

The second sentence sounds less direct or less certain. It describes an appearance or impression, not a definite fact.

-as marks the present tense in Esperanto.

So:

• ŝajni = to seem
• ŝajnas = seems / is seeming

Some common verb endings are:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command / wish

So ŝajnas tells you the action or state is happening now.

In Esperanto, after verbs like ŝajni and esti, you normally use an adjective if you are describing the subject.

Here, stranga describes la kaverno, so it must be an adjective.

• La kaverno ŝajnas stranga. = The cave seems strange.

An adverb in -e would describe the verb, not the cave itself, so that would mean something different.

Because there is no direct object here.

The -n ending is often used for the accusative, which usually marks the direct object. But in this sentence:

• la kaverno is the subject
• ŝajnas is the verb
• stranga is a predicate adjective describing the subject

So no accusative is needed.

Compare:

• Mi vidas la kavernon. = I see the cave.
  Here kavernon gets -n because it is the direct object.

But in:

• La kaverno ŝajnas stranga.

nothing is being directly acted upon, so there is no -n.

It actually already matches kaverno.

In Esperanto, adjectives agree with the nouns they describe in number and case. Here both are:

• singular
• not accusative

So the forms are:

• kaverno
• stranga

If the noun were plural, the adjective would also be plural:

• La kavernoj ŝajnas strangaj. = The caves seem strange.

If accusative were needed, both would take -n when appropriate.

Yes, Esperanto word order is fairly flexible, but the normal order here is very natural:

• La kaverno ŝajnas stranga.

You might also see:

• Stranga ŝajnas la kaverno.

That can sound more literary or emphatic, as if highlighting stranga first.

For learners, the basic order subject + verb + complement is the safest and most natural choice.

The letter ŝ is pronounced like English sh in ship.

So ŝajnas is approximately:

• SHAI-nahs

More exactly:

• ŝaj sounds like English shy
• nas sounds roughly like nahs

Esperanto stress usually falls on the second-to-last syllable, so in ŝajnas the stress is on ŝaj.

A rough English-friendly pronunciation is:

lah kah-VER-noh SHAI-nahs STRAHN-gah

A few notes:

• la = lah
• kaverno has stress on ver: kah-VER-noh
• ŝajnas has stress on ŝaj: SHAI-nahs
• stranga has stress on stran: STRAHN-gah

Esperanto spelling is very regular, so once you learn the sounds of the letters, pronunciation becomes much easier than in English.

It is often translated as cave or cavern depending on context. In many situations, either English word can work.

A learner should mainly notice that kaverno is a normal noun meaning an underground hollow space such as a cave. The exact English nuance depends on the context, not on any unusual grammar in this sentence.