La bestkuracisto steriligos la katon nur se la posedanto konsentos.

It is a compound noun:

• besto = animal
• kuracisto = doctor

So bestkuracisto literally means animal-doctor, i.e. veterinarian / vet.

In Esperanto, compounds are very common, and the last part is usually the main idea. So a bestkuracisto is a kind of kuracisto.

Steriligos can be divided like this:

• steril- = sterile
• -ig- = make something become
• -os = future tense

So steriligos means will make sterile, which in this context means will sterilize / will spay or neuter.

This is a very common Esperanto pattern:

• pura = clean
• purigi = to clean, to make clean

Likewise:

• sterila = sterile
• steriligi = to sterilize, to make sterile

Because la katon is the direct object of the verb steriligos.

In Esperanto, the direct object usually takes -n:

• La bestkuracisto = the veterinarian, the subject
• la katon = the cat, the thing being sterilized

So the -n tells you that the cat receives the action.

Esperanto does not follow the same rule as English here.

In Esperanto, after se, you can use the tense that matches the actual time of the action. Since the agreement is in the future, konsentos is perfectly normal.

So:

• se la posedanto konsentos = if the owner will agree / if the owner agrees later

English usually uses present tense after if, but Esperanto often uses future tense more directly.

Nur se means only if.

It introduces a necessary condition:

• La bestkuracisto steriligos la katon nur se la posedanto konsentos.
• The vet will sterilize the cat only if the owner agrees.

So the owner’s agreement is required.

This is different from just se, which means simply if.

Posedanto comes from:

• posedi = to own, to possess
• -ant- = someone doing the action
• -o = noun ending

So posedanto literally means one who owns, i.e. owner.

This is a very regular Esperanto pattern:

• lernanto = learner, student
• parolanto = speaker
• posedanto = owner

Because it refers to a specific, identifiable owner: the owner of this cat.

Esperanto uses la for definite reference, much like English the. Even if the owner has not been mentioned earlier, the context makes it clear which owner is meant.

Also, Esperanto has:

• la = the
• no separate word for a/an

So la posedanto means the owner.

Yes. Esperanto word order is fairly flexible, especially because the object is marked by -n.

For example, you could also say:

Nur se la posedanto konsentos, la bestkuracisto steriligos la katon.

That means the same thing: Only if the owner agrees will the vet sterilize the cat.

The original order is neutral and natural, but moving the nur se clause to the front gives it more emphasis.

Not always exactly.

In general, steriligi means to make sterile. In a veterinary context, that naturally means to spay or neuter.

So in this sentence, a learner should understand it as the normal animal-care meaning, not as something like disinfecting an object.

Context tells you which sense is meant.

The endings and structure tell you:

• La bestkuracisto is the subject, because it has no -n
• steriligos is the main verb
• la katon is the direct object, because it has -n
• nur se la posedanto konsentos is the condition clause

So the structure is:

[The vet] [will sterilize] [the cat] [only if the owner agrees].

Even if Esperanto word order can move around, these endings make the roles clear.