Ŝi esprimis sian opinion.

Because sia is the reflexive possessive in Esperanto.

In this sentence, the subject is ŝi (she), and the possessed thing—opinion—belongs to that same subject. So Esperanto uses sia:

• Ŝi esprimis sian opinion. = She expressed her own opinion.

If you said ŝian opinion, that would normally mean the opinion of some other female person, not the subject of the sentence.

Compare:

• Ŝi esprimis sian opinion. = She expressed her own opinion.
• Ŝi esprimis ŝian opinion. = She expressed another woman’s opinion.

Because opinion is the direct object of the verb esprimis, and in Esperanto the direct object usually takes -n.

Also, words that describe or go with a noun must agree with it. Since sian goes with opinion, it also takes -n.

So:

• opinio = opinion
• opinion = opinion (as a direct object)
• sia opinio = her own opinion
• sian opinion = her own opinion (as a direct object)

This agreement is very common in Esperanto.

-is is the Esperanto ending for the past tense.

So:

• esprimas = expresses / is expressing
• esprimis = expressed
• esprimos = will express

The verb root here is esprim-, meaning express.

The basic form is opinio.

Esperanto nouns normally end in -o:

• opinio = opinion

When a noun is the direct object, you add -n:

• opinion = opinion (object form)

So in this sentence, opinio becomes opinion because it is what she expressed.

Yes. Esperanto word order is fairly flexible because the -n ending helps show what the object is.

For example, these are possible:

• Ŝi esprimis sian opinion.
• Sian opinion ŝi esprimis.

Both mean essentially the same thing, though the second may sound more emphatic or stylistically marked.

The most neutral order is still subject + verb + object, as in the original sentence.

Because the phrase already has a possessive: sian opinion = her own opinion.

In Esperanto, as in English, you normally do not use la together with a possessive in this kind of phrase.

So you say:

• ŝia libro = her book
• sia opinio = her own opinion

not usually:

• la ŝia libro
• la sia opinio

A rough pronunciation guide for an English speaker is:

• Ŝi ≈ shee
• esprimis ≈ es-PREE-mis
• sian ≈ SEE-an
• opinion ≈ o-pee-nee-ON

A few details:

• ŝ sounds like English sh
• s is always like the s in see, never like z
• stress in Esperanto usually falls on the second-to-last syllable

So:

• es-PRI-mis
• SI-an
• o-pi-ni-ON

Yes, when the possessor is the subject of the clause, Esperanto normally uses sia.

Examples:

• Li amas sian hundon. = He loves his own dog.
• Ŝi perdis sian libron. = She lost her own book.
• Ili defendis sian ideon. = They defended their own idea.

But if the possessor is not the subject, then you use an ordinary possessive like lia, ŝia, ilia, etc.

This is one of the most important differences from English.

Usually yes. Esprimi is very similar to English express.

Common examples:

• esprimi opinion = express an opinion
• esprimi sentojn = express feelings
• esprimi sin = express oneself

So Ŝi esprimis sian opinion is a very natural Esperanto sentence.

Because Esperanto subjects do not need a special ending. The basic pronoun form is used for the subject:

• mi = I
• vi = you
• li = he
• ŝi = she

Only the direct object typically gets -n. So:

• Ŝi = she
• ŝin = her (as a direct object)

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

No. That would be incorrect.

The noun opinio already ends in -o. To make it a direct object, you replace the final -o with -on:

• opinio → opinion

You do not add an extra syllable like -on after the full word. So:

• correct: opinion
• incorrect: opinionon

The same pattern works for other nouns:

• libro → libron
• ideo → ideon
• demando → demandon