Mi malfermas la leterkeston, ĉar mi atendas leteron.

The -n ending marks the direct object in Esperanto.

In this sentence:

• Mi malfermas la leterkeston = I open the mailbox
  • la leterkeston is the thing being opened, so it gets -n
• mi atendas leteron = I am expecting a letter
  • leteron is the thing being expected, so it also gets -n

This is called the accusative ending.

A useful pattern is:

• mi vidas la hundon = I see the dog
• mi manĝas pomon = I eat an apple

The thing directly affected by the verb usually takes -n.

La means the.

So:

• la leterkeston = the mailbox
• leteron = a letter / some letter

Esperanto has:

• la for definite nouns
• no indefinite article for a/an

So English a letter is simply letero or, as a direct object here, leteron.

The speaker is talking about a specific mailbox, so la is used. But they are expecting a letter, not necessarily a specific one already identified, so la is not used.

Yes. Leterkesto is a compound noun.

It is made from:

• letero = letter
• kesto = box, chest, container

So leterkesto literally means letter-box, that is, a mailbox or letter box.

In Esperanto, compound words are very common. Usually, the final part is the main word, and the earlier part modifies it.

So here:

• kesto = the main idea, a box
• leter- = tells what kind of box

You will see this pattern often:

• libroŝranko = bookcase
• manĝotablo = dining table
• dormoĉambro = bedroom

The prefix mal- makes a word into its opposite.

So:

• fermi = to close
• malfermi = to open

That is one of the most useful features of Esperanto. Instead of learning completely unrelated opposite words, you can often use mal-.

Examples:

• bona = good → malbona = bad
• granda = big → malgranda = small
• varma = warm → malvarma = cold

So malfermas means opens or is opening.

Esperanto present tense in -as covers both simple present and present progressive, depending on context.

So mi malfermas can mean:

• I open
• I am opening

Likewise:

• mi atendas can mean I wait, I am waiting, I expect, or I am expecting, depending on context

Esperanto does not usually make a special grammatical distinction like English does between I open and I am opening. The context tells you which is meant.

Ĉar means because.

It introduces the reason for something:

• Mi malfermas la leterkeston, ĉar mi atendas leteron. = I open the mailbox because I am expecting a letter.

It connects two clauses:

1. Mi malfermas la leterkeston
2. mi atendas leteron

The second clause explains the first.

Also, note the spelling: ĉ is a separate letter in Esperanto, pronounced roughly like ch in church.

So ĉar is pronounced approximately like char.

Because ĉar introduces a subordinate clause, and Esperanto normally separates that clause with a comma.

So:

• Mi malfermas la leterkeston, ĉar mi atendas leteron.

This is very common in Esperanto. Commas are often used a bit more regularly than in English before subordinate clauses.

You will often see:

• Mi restas hejme, ĉar pluvas.
• Li ĝojas, ĉar ŝi venis.

Yes, Esperanto word order is fairly flexible, mainly because the -n ending shows the direct object.

For example, these are still understandable:

• Mi malfermas la leterkeston, ĉar mi atendas leteron.
• La leterkeston mi malfermas, ĉar mi atendas leteron.

Because leterkeston has -n, you still know it is the object.

However, the most neutral and common order is:

• subject + verb + object

So the original sentence is the most straightforward version for a learner.

Yes. Atendi commonly means to wait for or to expect, depending on context.

So:

• Mi atendas la buson = I am waiting for the bus
• Mi atendas leteron = I am expecting a letter / waiting for a letter

Unlike English, Esperanto atendi does not need a separate word like for before its object. The thing waited for is a direct object, so it takes -n.

That is why it is:

• mi atendas leteron not
• mi atendas por letero

Normally, no. In Esperanto, each clause usually states its subject explicitly unless the structure clearly avoids repetition in some other way.

Here there are two clauses:

• Mi malfermas la leterkeston
• ĉar mi atendas leteron

Each clause has its own verb:

• malfermas
• atendas

So each clause normally needs its own subject:

• mi
• mi

Leaving out the second mi would sound incomplete or unnatural to most speakers.

A few key pronunciation rules help here:

• Every vowel is pronounced clearly
• Stress is always on the second-to-last syllable

So:

• le-ter-KES-ton
• le-TE-ron
• mal-FER-mas
• a-TEN-das
• ĉar = roughly char

Also remember:

• e is like e in bet, not like English ee
• o is a clear o
• r is usually trilled or tapped, depending on the speaker

So Esperanto pronunciation is much more regular than English spelling.