Post longa serĉado ni malkovris, kial vi kaŝis la leteron inter la libroj.

Post is a preposition meaning after. The phrase post longa serĉado gives the time/background for the main action: after a long search.

So the structure is:

• post = after
• longa = long
• serĉado = searching, search as a process

It functions like an adverbial phrase telling us when or under what circumstances the discovery happened.

Because post is a preposition, and the noun after a preposition normally does not take the accusative -n.

Also, adjectives agree with their nouns, so:

• longa serĉado = both have no -n
• if the noun had -n, the adjective would also need it

Here the whole phrase is simply the object of the preposition post, so nominative-style endings are used.

This is about the suffix -ad-.

• serĉo = a search
• serĉado = searching, the activity/process of searching

The suffix -ad- often emphasizes duration, repetition, or the ongoing nature of an action. So longa serĉado suggests a prolonged search process, not just a single brief search.

Esperanto often builds words very logically.

• kovri = to cover
• mal- = the opposite
• malkovri = literally to uncover

From that literal sense, malkovri also means to discover or to find out.

So ni malkovris means we discovered / we found out.

In Esperanto, verbs do not change according to the subject.

The ending tells you the tense:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = imperative/jussive

So:

• mi malkovris
• ni malkovris
• vi malkovris
• ili malkovris

all use the same past-tense form malkovris. The subject pronoun tells you who did the action.

Kial is an interrogative/relative adverb meaning why.

In this sentence, it introduces an indirect question:

• direct question: Kial vi kaŝis la leteron?
• indirect question: Ni malkovris, kial vi kaŝis la leteron.

So it does not mean because. It means why, but inside a larger sentence.

The comma separates the main clause from the subordinate clause.

Main clause:

• Post longa serĉado ni malkovris

Subordinate clause:

• kial vi kaŝis la leteron inter la libroj

In Esperanto, a comma before a subordinate clause like this is very common and helps make the sentence structure clear.

Because la leteron is the direct object of kaŝis.

The thing being hidden is the letter, so it takes the accusative ending:

• la letero = the letter
• la leteron = the letter as the direct object

A very literal way to see it is: you hid what? → the letter

That is why -n appears there.

Here inter la libroj describes location: the letter was hidden among the books.

With prepositions in Esperanto, the accusative can sometimes show direction toward something, but if you are just describing where something is, you usually do not use -n.

So:

• inter la libroj = among the books, in that location
• inter la librojn could suggest movement into that position

Because this sentence is about where the letter was hidden, inter la libroj is the natural choice.

Esperanto often uses the simple past -is where English might use either hid, has hidden, or had hidden, depending on context.

So vi kaŝis simply marks the action as past. The exact time relationship is understood from the sentence as a whole.

If a speaker really wanted to emphasize had hidden, Esperanto could use a more explicit form, but very often the simple past is enough and sounds natural.

Yes, Esperanto has fairly flexible word order, because endings show grammatical roles clearly.

This sentence uses a neutral, natural order:

• time/background first: Post longa serĉado
• main statement: ni malkovris
• subordinate clause: kial vi kaŝis la leteron inter la libroj

You could move some parts around for emphasis, but the original order is straightforward and easy to understand.

It has three main parts:

1. Post longa serĉado
   a prepositional phrase giving background/time

2. ni malkovris
   the main clause

3. kial vi kaŝis la leteron inter la libroj
   an indirect-question subordinate clause explaining what we discovered

So the core pattern is:

• After a long search
  • we discovered
    • why you hid the letter among the books

That makes it a good example of how Esperanto combines a main clause with an indirect question.