Ŝi aĉetis eksterlandan biciklon.

Because it is the direct object of the verb aĉetis (bought).

In Esperanto, the direct object usually takes the ending -n. So:

• biciklo = a bicycle
• biciklon = a bicycle, as the thing being bought

In this sentence, she is doing the action, and the bicycle is receiving the action, so biciklon gets -n.

Because adjectives must agree with the nouns they describe.

Since biciklon is:

• singular
• adjective-compatible
• in the accusative case (-n)

the adjective must match it:

• eksterlanda biciklo = a foreign bicycle
• eksterlandan biciklon = a foreign bicycle, as the direct object

This agreement is a very important feature of Esperanto.

Aĉetis is the verb aĉeti (to buy) in the past tense.

Esperanto verb endings are very regular:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command/jussive
• -i = infinitive

So:

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

Ŝi means she.

Pronunciation:

• ŝ sounds like English sh
• i sounds like ee

So ŝi is pronounced roughly like shee.

Esperanto has separate pronouns such as:

• mi = I
• vi = you
• li = he
• ŝi = she
• ĝi = it
• ni = we
• ili = they

Esperanto does have a definite article: la = the.

But it has no indefinite article like English a/an.

So:

• biciklo can mean a bicycle or just bicycle depending on context
• la biciklo means the bicycle

In this sentence, biciklon is naturally understood as a bicycle unless the context suggests a specific one.

Eksterlandan comes from eksterlanda, meaning foreign or from another country.

It is built from parts:

• ekster = outside
• lando = country
• -a = adjective ending

So the idea is something like outside-country-related → foreign.

In the sentence, it describes the bicycle as being foreign or from abroad.

In Esperanto, adjectives can come before or after the noun.

So both are possible:

• eksterlandan biciklon
• biciklon eksterlandan

The first order is much more common and sounds more natural in ordinary usage.

Because Esperanto uses endings like -a and -n, word order is more flexible than in English.

Yes. Esperanto allows more flexible word order because grammatical roles are shown by endings.

For example, these all keep basically the same meaning:

• Ŝi aĉetis eksterlandan biciklon.
• Eksterlandan biciklon ŝi aĉetis.
• Aĉetis ŝi eksterlandan biciklon.

However, the normal, neutral order is:

subject + verb + object

So the original sentence is the most straightforward version.

You can identify them by form:

• Ŝi is the subject pronoun: the person doing the action
• biciklon has -n, so it is the direct object: the thing being bought

This is one of the strengths of Esperanto: endings make sentence structure very clear.

The dictionary forms are:

• ŝi = she
• aĉeti = to buy
• eksterlanda = foreign
• biciklo = bicycle

In the sentence, some of them are changed for grammar:

• aĉeti → aĉetis for past tense
• eksterlanda → eksterlandan to match an accusative singular noun
• biciklo → biciklon for the direct object

It is singular.

In Esperanto:

• singular nouns have no special plural ending
• plural nouns take -j

So:

• biciklo = bicycle
• biciklon = bicycle, direct object
• bicikloj = bicycles
• biciklojn = bicycles, direct object

Since there is no -j here, it is singular.

You would add la:

Ŝi aĉetis la eksterlandan biciklon.

Here:

• la = the
• eksterlandan still matches biciklon
• biciklon still keeps -n because it is still the direct object

Pronouns in Esperanto have their own system.

The subject form is:

• ŝi = she

If it were a direct object, it would become:

• ŝin = her

So pronouns can also take -n, but they do not use noun endings like -o or adjective endings like -a.

In this sentence, ŝi is the subject, so it stays ŝi.

A simple pronunciation guide is:

SHEE ah-CHEH-tis eks-ter-LAHN-dan bi-TSI-klon

A few useful details:

• ŝ = sh
• ĉ = ch as in church
• c = ts
• stress is usually on the second-to-last syllable

So:

• Ŝi → SHEE
• aĉetis → ah-CHEH-tis
• eksterlandan → eks-ter-LAHN-dan
• biciklon → bi-TSI-klon

You only need to change the verb ending:

• Ŝi aĉetas eksterlandan biciklon. = She buys / is buying a foreign bicycle.
• Ŝi aĉetos eksterlandan biciklon. = She will buy a foreign bicycle.

The rest of the sentence stays the same unless some other grammatical role changes.