En la malnova skatolo mi malkovris du fotojn kaj unu leteron.

Because en here shows a location, not movement.

• en la malnova skatolo = in the old box
• en la malnovan skatolon would suggest movement into the old box

In Esperanto, the -n ending can mark direction with some prepositions. Since the sentence says where the discovery happened, skatolo stays without -n.

la is the definite article, meaning the.

So:

• la skatolo = the box
• skatolo = a box or just box, depending on context

Esperanto has la, but it does not have a separate word for a/an.
So la malnova skatolo means the old box.

mal- is a very common Esperanto prefix meaning the opposite of.

• nova = new
• malnova = old

So malnova skatolo literally means something like not-new box, but the natural English meaning is simply old box.

Because adjectives in Esperanto normally end in -a.

• malnova = old
• granda = big
• bela = beautiful

Here malnova describes skatolo, so it uses the adjective ending -a.

Esperanto word order is more flexible than English word order.

This sentence starts with the location for emphasis:

• En la malnova skatolo mi malkovris... = In the old box I discovered...

You could also say:

• Mi malkovris du fotojn kaj unu leteron en la malnova skatolo.

Both are correct. The version with En la malnova skatolo first gives a little more attention to the place.

malkovris is the past tense form of malkovri.

Breakdown:

• malkovri = to discover
• -is = past tense

So:

• mi malkovris = I discovered

The ending -is always marks the past tense in Esperanto.

Yes, historically and structurally it does.

• kovri = to cover
• malkovri = to uncover, and by extension to discover

In actual use, malkovri is the normal word for discover.
So in this sentence, it is best understood simply as discovered, not necessarily physically uncovering something.

Because fotojn is both:

1. plural → -j
2. the direct object → -n

So:

• foto = photo
• fotoj = photos
• fotojn = photos as a direct object

Since I discovered two photos, two photos is what was discovered, so it gets the accusative -n.

For the same reason: leteron is also a direct object.

• letero = letter
• leteron = letter as a direct object

So:

• mi malkovris unu leteron = I discovered one letter

Because it is singular, it has only -n, not -jn.

Because numerals in Esperanto are usually invariable. They do not normally take endings for plural or accusative.

So you say:

• du fotoj
• du fotojn
• unu letero
• unu leteron

The noun changes when needed, but the numeral usually stays the same.

Esperanto does not have an indefinite article.

English distinguishes:

• a letter
• the letter

Esperanto only has a word for the, which is la.

So:

• unu letero can mean one letter
• letero can mean a letter or just letter, depending on context
• la letero means the letter

In this sentence, unu leteron is enough.

Because Esperanto matches the noun to the number being counted.

• unu letero = one letter
• du fotoj = two photos

So with one, the noun is singular; with numbers above one, the noun is plural.

In this sentence:

• du fotojn = two photos
• unu leteron = one letter

Yes. Esperanto allows some freedom in word order, especially because the -n ending helps show what the direct object is.

For example, these are all possible:

• En la malnova skatolo mi malkovris du fotojn kaj unu leteron.
• Mi malkovris du fotojn kaj unu leteron en la malnova skatolo.

The basic meaning stays the same, but the emphasis changes a little. Starting with En la malnova skatolo highlights the location first.