Ŝi skribas pri sia vivo.

Because sia is the reflexive possessive in Esperanto.

Use sia when the thing being possessed belongs to the subject of the sentence. Here, the subject is ŝi (she), so sia vivo means her own life.

• Ŝi skribas pri sia vivo. = She writes about her own life.
• Ŝi skribas pri ŝia vivo. = She writes about another woman’s life.

This is one of the most important differences between Esperanto and English, since English her can mean either one.

Pri is a preposition meaning about, concerning, or regarding.

So skribi pri io means to write about something.

Examples:

• Li parolas pri la libro. = He talks about the book.
• Mi pensas pri vi. = I think about you.

In this sentence, pri sia vivo is a prepositional phrase meaning about her own life.

In Esperanto, all present-tense verbs end in -as.

So:

• skribi = to write
• skribas = writes / is writing

The verb ending does not change depending on the subject.

So all of these use -as:

• mi skribas = I write
• vi skribas = you write
• ŝi skribas = she writes
• ili skribas = they write

This is much simpler than English, where we have I write but she writes.

Sia is a possessive adjective.

It works like other adjective-like possessives such as:

• mia = my
• via = your
• lia = his
• ŝia = her
• nia = our
• ilia = their

But sia is special because it refers back to the subject.

Since it is adjective-like, it has the -a ending and can agree with the noun it modifies:

• sia vivo = her own life
• siaj vivoj = her own lives
• siajn vivojn = her own lives, as direct object

In your sentence, vivo is singular and not accusative, so sia stays just sia.

Because -o is the normal ending for a noun in Esperanto.

So:

• vivo = life
• libro = book
• domo = house

In this sentence, vivo is the noun inside the phrase pri sia vivo.

Also notice that it does not take -n, because it is not the direct object of the verb. It comes after the preposition pri, so the basic noun form is used.

Because there is no direct object here.

The verb skribi often can take a direct object:

• Ŝi skribas leteron. = She writes a letter.

There, leteron gets -n because it is the direct object.

But in Ŝi skribas pri sia vivo, the phrase pri sia vivo is introduced by the preposition pri. That means vivo is part of a prepositional phrase, not a direct object, so there is no accusative -n.

A helpful approximate pronunciation is:

shii SKRI-bahs pree SEE-ah VEE-vo

A few important points:

• ŝ sounds like English sh
• i sounds like ee
• a sounds like ah
• o sounds like oh

Stress in Esperanto normally falls on the second-to-last syllable of each word:

• Ŝi — one syllable
• SKRI-bas
• pri — one syllable
• SI-a
• VI-vo

Yes, Esperanto word order is fairly flexible, though Ŝi skribas pri sia vivo is the most neutral and natural order.

You could also say:

• Pri sia vivo ŝi skribas.

That puts more emphasis on pri sia vivo.

Even when word order changes, the meaning usually stays clear because Esperanto relies a lot on endings and prepositions, not only word position.

Still, for learners, subject + verb + other information is usually the safest and most natural pattern.

No. Ŝi is capitalized here only because it is the first word of the sentence.

Normally, it is written ŝi in the middle of a sentence:

• Mi vidas, ke ŝi skribas.

So capitalization here is just normal sentence capitalization, not something special about pronouns.

Yes. The Esperanto present tense -as can cover both ideas, depending on context.

So ŝi skribas can mean:

• she writes
• she is writing

If you need to make the time sense clearer, the surrounding context usually does that.

For example:

• Nun ŝi skribas pri sia vivo. = Now she is writing about her life.
• Ofte ŝi skribas pri sia vivo. = She often writes about her life.

Sia vivo specifically means her own life.

If you said pri la vivo, that would usually mean about life in a general sense, not specifically her life.

Compare:

• Ŝi skribas pri sia vivo. = She writes about her own life.
• Ŝi skribas pri la vivo. = She writes about life.

So sia makes the phrase personal and specific.