La spino estas pli fleksebla, ekde kiam ŝi komencis la ekzercon.

Spino means spine, backbone, or vertebral column.

So this sentence is specifically talking about the spine, not the whole back in a general sense.

• spino = spine/backbone
• dorso = back

If you wanted to say Her back is more flexible, Esperanto might more naturally use dorso, depending on the exact meaning.

La spino means the spine. Esperanto often uses la when the thing is understood from context.

With body parts, Esperanto can sometimes use la where English might use a possessive like her or his, especially when the owner is obvious.

So depending on context:

• la spino = the spine
• ŝia spino = her spine

If the sentence is about her and her own exercise, many learners might expect ŝia spino. That would also be possible if you want to make the possessor explicit. Using la spino can sound more neutral or more context-dependent.

Because fleksebla is an adjective describing spino, and here it is the complement of estas.

Breakdown:

• spin-o = noun
• fleksebla = adjective, singular, no accusative

The adjective matches the noun in number and case:

• singular noun without -n → singular adjective without -n
• la spino estas fleksebla = the spine is flexible

You would only add -n to the adjective if the noun were also in the accusative:

• Mi vidas la flekseblan spinon = I see the flexible spine

But in your sentence, la spino is the subject, so no -n.

Pli means more. It does not change form.

So:

• fleksebla = flexible
• pli fleksebla = more flexible

This is the normal Esperanto way to make a comparative:

• granda = big
• pli granda = bigger / more big
• rapide = quickly
• pli rapide = more quickly

Because the sentence does not explicitly say more flexible than what.

In Esperanto:

• pli ... ol ... = more ... than ...
• pli fleksebla ol antaŭe = more flexible than before

But here the comparison point is understood from the second part of the sentence:

• ekde kiam ŝi komencis la ekzercon = since she started the exercise

So the meaning is basically more flexible than before she started. Since that comparison is implied, ol is not necessary.

Because the sentence describes her spine's current state.

• estas = is
• The sentence says that now, at the present time, the spine is more flexible.

In English, people often say something like:

• Her spine has become more flexible since she started the exercise.

Esperanto often expresses this kind of idea with a simple present-state sentence:

• La spino estas pli fleksebla...

So the focus is not the change itself, but the result that is true now.

Ekde kiam means since in the sense of from the time when.

Literally:

• ekde = starting from
• kiam = when

So:

• ekde kiam ŝi komencis la ekzercon = since she started the exercise

Using just kiam would mean only when, not since:

• kiam ŝi komencis la ekzercon = when she started the exercise

That would not express the continuing time relationship.

Also, many speakers use de kiam for this idea as well. Ekde kiam adds a stronger sense of starting from that point onward.

Because she started the exercise at some point in the past.

• komenci = to begin / to start
• komencis = began / started

This past action serves as the starting point for the present situation:

• past event: she started the exercise
• present result: the spine is now more flexible

So the tense sequence is very natural.

Because la ekzercon is the direct object of komencis.

She started what?

• la ekzercon

That makes it the object, so it takes the accusative -n:

• ŝi komencis la ekzercon

Without -n, it would be ungrammatical in normal Esperanto.

Grammatically, la ekzercon means the exercise.

That usually suggests a specific exercise already known from context:

• a particular exercise
• a specific exercise routine
• perhaps a prescribed exercise from an earlier sentence

If you wanted a more general sense, other wording might be possible depending on what you mean:

• ekzerci sin = to exercise
• komencis ekzerci sin = started exercising

So this sentence, as written, sounds more like she started the exercise or the exercise program, not just exercise in general.

Because that part is a subordinate time clause, and Esperanto often separates such clauses with a comma.

So the structure is:

• main clause: La spino estas pli fleksebla
• subordinate clause: ekde kiam ŝi komencis la ekzercon

The comma helps readability. In practice, Esperanto punctuation is often fairly clear and logical, and many writers would include this comma here.

Yes. You could also say:

Ekde kiam ŝi komencis la ekzercon, la spino estas pli fleksebla.

That has essentially the same meaning.

The difference is mostly emphasis:

• La spino estas pli fleksebla... starts with the result
• Ekde kiam ŝi komencis la ekzercon... starts with the time frame

Both are normal Esperanto.