La du amikoj sidas aparte.

La makes the noun phrase definite: it points to a specific pair of friends, not just any two friends.

• La du amikoj = the two friends
• Du amikoj = two friends / two friends in general

So the version with la is used when the speaker and listener already know which two friends are meant, or when the context makes them identifiable.

In Esperanto, nouns end in -o, and the plural is made with -j.

• amiko = friend
• amikoj = friends

Because there are two friends, the noun has to be plural: du amikoj.

The number du stays the same. Numbers in Esperanto do not normally take plural endings when they are used this way.

So you say:

• du amikoj
• tri libroj
• kvar seĝoj

The plural marking goes on the noun, not on the number.

The ending -as marks the present tense in Esperanto.

• sidas = sits / are sitting
• sidis = sat / were sitting
• sidos = will sit / will be sitting

So sidas tells you the action is happening in the present.

No. In Esperanto, verbs do not change according to person or number.

That means the same verb form is used with:

• mi sidas
• vi sidas
• li sidas
• ili sidas

So even though la du amikoj is plural, the verb is still simply sidas.

Because aparte is an adverb, and it modifies the verb sidas.

In Esperanto:

• -a = adjective, used to describe a noun
• -e = adverb, used to describe a verb, adjective, or another adverb

Here, aparte tells us how they are sitting, so an adverb is needed.

• aparte = separately / apart
• aparta = separate, distinct

For example:

• La du amikoj sidas aparte.
  The adverb describes the sitting.

• Ili estas apartaj.
  Here apartaj is an adjective describing ili.

Because there is no direct object.

• La du amikoj is the subject.
• sidas is an intransitive verb here.
• aparte is an adverb.

The accusative -n is mainly used for a direct object, and this sentence does not have one.

If you had a direct object, then you would see -n, for example:

• Mi vidas la du amikojn.

But in La du amikoj sidas aparte, nothing is being directly acted on.

Yes, Esperanto word order is fairly flexible, because the endings show the grammatical roles.

The neutral, most straightforward order is:

• La du amikoj sidas aparte.

But other orders are possible for emphasis, such as:

• Aparte sidas la du amikoj.

That sounds more marked or literary, with extra focus on aparte. For a learner, the original order is the safest and most natural choice.

Because du is a cardinal number, meaning two.

• du = two
• dua = second

So:

• la du amikoj = the two friends
• la dua amiko = the second friend

This is a very common distinction in Esperanto:

• plain number = du
• ordinal number = dua

In this sentence, it most naturally means that the two friends are sitting separately, not together.

But aparte can also be used more broadly depending on context, for example to mean something like separately, apart, or sometimes individually.

Here, because it is used with sidas, the physical sense is the most natural one.