Sur la tablo kuŝas la sendota letero apud la jam aĉetita poŝtmarko.

Esperanto word order is fairly flexible. The sentence starts with the location, Sur la tablo (On the table), and then uses the verb before the subject:

Sur la tablo kuŝas la sendota letero...

This is a natural way to present a scene, similar to English On the table lies the letter...

So the order is not unusual; it simply puts the place first for emphasis. The subject is still la sendota letero.

Kuŝi means to lie, to be lying, or to be in a lying position. It is often used for things resting flat or horizontally.

So:

• La letero estas sur la tablo = The letter is on the table
• La letero kuŝas sur la tablo = The letter is lying on the table

Both are possible, but kuŝas is more vivid and natural when talking about an object resting on a surface.

Sendota is a participle-based adjective from sendi (to send).

It breaks down like this:

• send- = send
• -ot- = future passive participle
• -a = adjective ending

So sendota means:

• to be sent
• going to be sent
• destined to be sent

Therefore, la sendota letero means the letter that is to be sent.

This is about active vs passive participles.

• sendonta = about to send
• sendota = about to be sent

A letter does not do the sending; it receives the action. So the passive form is needed:

• la sendota letero = the letter to be sent

If you said la sendonta letero, that would suggest the letter is going to send something, which does not make sense here.

Aĉetita comes from aĉeti (to buy).

It breaks down like this:

• aĉet- = buy
• -it- = past passive participle
• -a = adjective ending

So aĉetita means bought.

Thus:

la jam aĉetita poŝtmarko = the already bought stamp
More natural English: the stamp already bought or the stamp that has already been bought

Jam means already. It modifies the idea of the stamp’s having been bought.

So:

• la jam aĉetita poŝtmarko = the already-bought stamp

Placing jam before the participle is very natural in Esperanto. It works much like an adverb before an adjective-like participle phrase.

Because in this sentence they are functioning as adjectives modifying nouns:

• sendota modifies letero
• aĉetita modifies poŝtmarko

In Esperanto, adjectives end in -a.

So even though these words come from verbs, once they describe nouns, they take the adjective ending:

• sendota letero
• aĉetita poŝtmarko

There is no direct object here, so no accusative is needed.

The sentence is basically saying that something is located somewhere:

• Sur la tablo = location
• kuŝas = lies
• la sendota letero = subject
• apud la jam aĉetita poŝtmarko = another location phrase

Also, with prepositions like sur and apud, you normally do not add -n when talking about location.

Compare:

• sur la tablo = on the table (location)
• sur la tablon = onto the table (motion toward)

Here the letter is already there, so sur la tablo is correct.

They are prepositions of place.

• sur = on
• apud = beside, next to

So:

• Sur la tablo = On the table
• apud la jam aĉetita poŝtmarko = beside the already bought stamp

This tells us where the letter is located.

La is Esperanto’s definite article, meaning the.

So:

• la sendota letero = the letter to be sent
• la jam aĉetita poŝtmarko = the stamp already bought

Esperanto has only one article, la. There is no separate word for a/an. If the noun were indefinite, you would usually just leave the article out:

• sendota letero = a letter to be sent
• aĉetita poŝtmarko = a bought stamp

Here both items are presented as specific ones.

Yes. Poŝtmarko means postage stamp or mail stamp.

It is made from:

• poŝto = post, mail
• marko = mark

So in this sentence it means the stamp that has already been bought for the letter.

Yes, that is often a very helpful way to understand them.

For example:

• la sendota letero
  = the letter that is to be sent

• la jam aĉetita poŝtmarko
  = the stamp that has already been bought

This is one reason participles are so useful in Esperanto: they let you express ideas compactly without needing a full clause every time.