La leterkesto estas tro malgranda por la granda pako, kiun mi volas sendi.

Esperanto very often builds new words by combining roots.

• leter- = letter
• kesto = box

So leterkesto literally means letter-box, i.e. mailbox. This kind of compound word is extremely common in Esperanto.

Because both noun phrases are definite:

• la leterkesto = the mailbox
• la granda pako = the large package

Esperanto uses la much like English the. If both things are specific, both can take la.

Because -a is the adjective ending in Esperanto.

• granda = big, large
• malgranda = small

Adjectives use -a and describe nouns. Here:

• malgranda describes leterkesto
• granda describes pako

mal- makes the opposite of a word.

• granda = big
• malgranda = small

This is a very common Esperanto pattern. So instead of learning completely separate unrelated words, you often learn a word and its opposite with mal-.

Because tro means too, while tre means very.

• tre malgranda = very small
• tro malgranda = too small

In this sentence, the mailbox is not just small; it is smaller than is acceptable for the package, so tro is the correct word.

Here por means something like for in the sense of suitable for or big enough for.

So tro malgranda por la granda pako means the mailbox is too small for that package, or more naturally, too small to fit that package.

Because por is a preposition, and nouns after prepositions normally stay in the basic form, without -n.

So:

• por la granda pako = correct

The accusative -n is not used here just because the noun comes after por.

Because kiun mi volas sendi is a subordinate relative clause, and Esperanto normally separates subordinate clauses with a comma.

So the comma marks the beginning of the clause that describes pako:

• la granda pako, kiun mi volas sendi

Because kiun is the direct object of sendi inside the relative clause.

Think of the clause by itself:

• Mi volas sendi la pakon = I want to send the package

Since the package is the object, it would be pakon with -n. When that noun is replaced by the relative word, it becomes kiun.

So:

• kiun mi volas sendi = that/which I want to send

It refers to la granda pako.

So the structure is:

• la granda pako = the large package
• kiun mi volas sendi = that I want to send

Together: the large package that I want to send.

Because they belong to different parts of the sentence and have different grammatical jobs.

• pako is inside the phrase por la granda pako, after the preposition por, so it stays without -n
• kiun is the direct object of sendi in the relative clause, so it takes -n

This is a very important Esperanto idea: a relative pronoun gets its case from its role in its own clause, not from the form of the noun it refers to.

Because Esperanto does not use a separate word like English to before the infinitive.

The infinitive already has the ending -i:

• sendi = to send
• voli = to want

So:

• mi volas sendi = I want to send

That is the normal Esperanto pattern.

Not necessarily. Leterkesto is perfectly understandable and literally means letter-box. Depending on context, you may also see poŝtkesto for a mailbox or postbox.

So this sentence is fine, but in real usage the exact word can vary a bit depending on what kind of mailbox is meant.