Mia onklino aĉetis bluan koverton kaj du poŝtmarkojn en la poŝtejo.

Because mia onklino is the subject of the sentence: she is the one doing the action.

In Esperanto, the direct object gets -n, but the subject does not.

• mia onklino = my aunt as the subject
• mian onklinon = my aunt as the direct object

So here, Mia onklino aĉetis... means My aunt bought..., and no -n is needed on onklino.

-is is the Esperanto ending for the past tense.

So:

• aĉeti = to buy
• aĉetas = buys / is buying
• aĉetis = bought
• aĉetos = will buy

This is one of the nice regular features of Esperanto: verb endings do not change for person.

• mi aĉetis = I bought
• ŝi aĉetis = she bought
• ili aĉetis = they bought

Because koverton is a singular direct object, so it takes -n.

Also, adjectives in Esperanto must agree with the nouns they describe in both number and case. Since koverton has -n, the adjective bluan must also have -n.

• blua koverto = a blue envelope
• bluan koverton = a blue envelope as a direct object

So both words change:

• blua → bluan
• koverto → koverton

Because the phrase is a plural direct object.

After a number like du, the noun is normally plural:

• du poŝtmarkoj = two stamps

But in this sentence, the stamps are also the direct object of aĉetis, so they also need -n:

• du poŝtmarkojn = two stamps as the thing bought

So the noun gets both endings:

• -j for plural
• -n for direct object

That gives poŝtmarkojn.

Because ordinary numerals like unu, du, tri usually stay the same in normal use.

The noun changes for plural and case, but the number itself usually does not:

• du libroj = two books
• mi vidas du librojn = I see two books

So in your sentence:

• du stays du
• poŝtmarko becomes poŝtmarkojn

Because here en shows location, not motion toward a place.

• en la poŝtejo = in / at the post office

The sentence says where the buying happened, not that someone moved into the post office.

If you wanted to express movement into the place, you could use -n:

• Ŝi iris en la poŝtejon. = She went into the post office.

So:

• en la poŝtejo = at/in the post office
• en la poŝtejon = into the post office

Esperanto has a definite article, la, but it has no indefinite article like English a/an.

So:

• koverto can mean an envelope or envelope, depending on context
• la koverto = the envelope

In this sentence:

• bluan koverton = a blue envelope
• la poŝtejo = the post office

The speaker is treating the post office as a specific, identifiable place.

Because possessive words like mia, via, lia, ŝia usually make la unnecessary.

So Esperanto normally says:

• mia onklino = my aunt
• via libro = your book

Not usually:

• la mia onklino

The possessive already makes the noun definite enough.

Poŝtejo is a good example of Esperanto word-building.

It can be broken down like this:

• poŝt- = post / mail
• -ej- = place used for something
• -o = noun ending

So poŝtejo literally means a place for mail, which is why it means post office.

The suffix -ej- is very common:

• lernejo = school, a place for learning
• vendejo = shop, a place for selling
• kuirejo = kitchen, a place for cooking

The main special letters here are ĉ and ŝ.

• ĉ sounds like ch in church
• ŝ sounds like sh in shoe

So:

• aĉetis sounds roughly like ah-CHE-tis
• poŝtmarkojn begins with a sh sound in poŝt-

Also, Esperanto stress is very regular: it falls on the second-to-last syllable.

Examples:

• on-KLI-no
• a-ĈE-tis
• ko-VER-ton
• poŝ-TE-jo

Not completely. Esperanto word order is fairly flexible because endings show the grammatical roles.

The neutral, most natural order here is:

• Mia onklino aĉetis bluan koverton kaj du poŝtmarkojn en la poŝtejo.

But because koverton and poŝtmarkojn have -n, you could rearrange things for emphasis:

• Bluan koverton kaj du poŝtmarkojn mia onklino aĉetis en la poŝtejo.

That said, beginners should usually stick to the normal subject–verb–object order unless there is a reason to emphasize something.