Ŝi etendas la spinon kaj kontrolas sian sintenon.

Because -as is the Esperanto ending for the present tense.

• etendas = stretches / is stretching
• kontrolas = checks / is checking

Unlike English, Esperanto verbs do not change for person:

• mi etendas = I stretch
• ŝi etendas = she stretches
• ili etendas = they stretch

The verb ending stays the same; only the subject changes.

Because the same subject, ŝi, applies to both verbs.

So this structure means:

• Ŝi etendas ... kaj kontrolas ...
• literally: She stretches ... and checks ...

Esperanto often omits repeating the subject when it is still clearly the same one.

The -n marks the direct object in Esperanto.

Here:

• etendas la spinon = stretches the spine
• kontrolas sian sintenon = checks her own posture

The things being stretched and checked are the direct objects, so they take -n.

Without -n, the sentence would be ungrammatical or would mean something different.

Because sia is the reflexive possessive. It means his/her/their own when it refers back to the subject of the clause.

In this sentence, the subject is ŝi, and the posture belongs to that same person, so Esperanto uses sian:

• Ŝi kontrolas sian sintenon = She checks her own posture.

If it were someone else’s posture, you would use ŝian:

• Ŝi kontrolas ŝian sintenon = She checks her posture, meaning another female person’s posture.

This is one of the most important differences between English and Esperanto.

Because sia is acting like an adjective, and adjectives in Esperanto agree with the noun they describe.

The noun is sintenon, which is:

• singular
• accusative (because it is a direct object)

So the possessive adjective must match it:

• sia sinteno = her own posture
• sian sintenon = her own posture as a direct object

So the -n on sian is there for agreement with sintenon.

Because Esperanto often uses the definite article with body parts when the owner is obvious from context.

So:

• Ŝi etendas la spinon naturally means She stretches her spine/back

This works much like other common Esperanto expressions:

• Mi lavas la manojn = I wash my hands
• Li fermis la okulojn = He closed his eyes

Using sian spinon would also be understandable, but la spinon is often more natural when it is obviously her own body.

In Esperanto, you normally do not use la together with possessive words like:

• mia
• via
• lia
• ŝia
• ĝia
• nia
• ilia
• sia

So Esperanto prefers:

• sia sinteno not
• la sia sinteno

The possessive already makes the noun definite enough.

It can mean either one.

Esperanto -as is just the present tense. It does not force the distinction that English often makes between:

• she stretches
• she is stretching

Context tells you which English translation sounds best.

So Ŝi etendas la spinon kaj kontrolas sian sintenon could describe:

• a habitual action, or
• something happening right now

depending on context.

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

This sentence uses the most neutral order:

• Subject + Verb + Object

So:

• Ŝi etendas la spinon kaj kontrolas sian sintenon.

You could rearrange parts for emphasis, though the neutral version is usually best for learners.

For example, you could change the order of the two actions:

• Ŝi kontrolas sian sintenon kaj etendas la spinon.

That would still be grammatical, but it would present the checking first.

A rough guide:

• ŝi sounds like shee
• kaj sounds like kai in sky without the s
• ŝ = English sh
• j = English y sound

So the whole sentence is approximately:

• SHEE eh-TEN-das la SPEE-non kai kon-TRO-las SEE-an sin-TE-non

Also remember that Esperanto stress normally falls on the second-to-last syllable:

• eTENdas
• spiNON
• konTROlas
• sinTEnon