Ŝi komencis novan karieron.

Ŝi means she. It is the third-person singular pronoun for a female person.

• ŝi = she
• li = he
• ĝi = it

So in Ŝi komencis novan karieron, ŝi is the subject: the person doing the action.

The letter ŝ is a separate Esperanto letter, pronounced like English sh in she.

So:

• ŝi is pronounced roughly like she
• plain s would sound different

Esperanto uses several letters with diacritics, and they are considered normal letters, not optional decorations.

Komencis is the past tense form of komenci (to begin / to start).

Esperanto verb endings are very regular:

• -i = infinitive → komenci = to begin
• -as = present → komencas = begins / is beginning
• -is = past → komencis = began / started
• -os = future → komencos = will begin

So Ŝi komencis means She began or She started.

It is novan because the noun it describes, karieron, is the direct object, and in Esperanto direct objects usually take -n. Adjectives agreeing with that noun also take -n.

So:

• nova kariero = a new career
• novan karieron = a new career, as the direct object

Both words get the object marking:

• nova → novan
• kariero → karieron

This is called agreement.

The -n marks the direct object: the thing directly affected by the verb.

In this sentence, what did she start?
She started a new career.

So karieron is the direct object of komencis, and that is why it ends in -n.

Because adjectives in Esperanto agree with the nouns they describe in both:

• number: singular or plural
• case: whether they have the accusative -n

Here:

• karieron is singular and accusative
• so the adjective must match: novan

This agreement helps show clearly which words belong together.

The dictionary form is kariero, which means career.

Esperanto nouns normally end in -o:

• kariero = career
• karieron = career as a direct object

So the -n is added to the basic noun form for grammar, not because the word itself changes meaning.

Yes. Because Esperanto marks the direct object with -n, word order is more flexible than in English.

For example, these all mean basically the same thing:

• Ŝi komencis novan karieron.
• Novan karieron ŝi komencis.
• Komencis ŝi novan karieron.

However, the most neutral and common order is still subject–verb–object, which is the version you have here.

Esperanto has only one article: la, which means the.

There is no separate word for English a or an. So:

• kariero can mean a career or just career, depending on context
• la kariero means the career

That is why novan karieron can naturally mean a new career without any extra word.

After the verb komenci, the noun is an object, not a subject complement.

Compare:

• Ŝi komencis novan karieron. = She started a new career.
  Here, novan karieron is the thing she started, so it takes -n.

But with a verb like esti (to be), you would not normally use the accusative:

• Ŝi estas nova instruistino. = She is a new teacher.

So the form depends on the role of the noun phrase in the sentence.

You only need to change the verb ending:

• Ŝi komencas novan karieron. = She is starting / starts a new career.
• Ŝi komencos novan karieron. = She will start a new career.

The rest stays the same because novan karieron is still the direct object.

A rough pronunciation is:

SHEE ko-men-TSIS NO-van ka-ri-EH-ron

A few helpful points:

• ŝi sounds like English she
• c in Esperanto is always like ts
• j sounds like English y
• stress usually falls on the second-to-last syllable

So:

• komencis → ko-MEN-cis
• karieron → ka-ri-E-ron, with stress on E

Yes. In many contexts, komenci can be translated as either begin or start.

So:

• Ŝi komencis novan karieron. can be understood as
• She began a new career.
• She started a new career.

English may prefer one or the other depending on style, but Esperanto komenci works well for both ideas here.