huninkyoseisyuzyutu ga owatta.

It is a compound noun meaning spay/neuter surgery.

A rough breakdown is:

• 不妊 = infertility / sterilization
• 去勢 = castration / neutering
• 手術 = surgery / operation

Put together, 不妊去勢手術 is a general veterinary-style term for the surgery that prevents reproduction. In practice, it often covers both spaying and neutering as one category.

が marks 不妊去勢手術 as the subject of the verb 終わった.

The verb 終わる is an intransitive verb, meaning it describes something ending or being over, rather than someone actively finishing it.

So:

• 不妊去勢手術が終わった。 = The spay/neuter surgery ended / is over.

The sentence focuses on the event itself being finished, not on who finished it.

This is an important distinction:

• 終わる = to end, to be over (intransitive)
• 終える = to finish something (transitive)

So:

• 手術が終わった = The surgery ended / is over
• 手術を終えた = Someone finished the surgery

In your sentence, が and 終わった go together naturally because the surgery is being treated as the thing that has come to an end.

終わった is the plain past form of 終わる. Here it means the surgery has ended or is over now.

In Japanese, the past form often covers both:

• a simple past event: ended
• a present-result meaning: is finished now

So depending on context, this could sound like:

• The surgery ended.
• The surgery is over.
• The surgery has finished.

All are reasonable translations.

Yes. 終わった is plain-form Japanese, so it sounds casual or neutral.

A polite version would be:

• 不妊去勢手術が終わりました。

That would be more natural in polite conversation, with a customer, or in a formal setting.

Japanese often omits information that is understood from context.

This sentence does not explicitly say:

• whose surgery it was
• what animal it was
• who is speaking
• who performed the surgery

So depending on context, it might mean something like:

• The cat’s spay surgery is over.
• The dog’s neutering is finished.
• The operation is done.

All of that extra information can be left unsaid if both speaker and listener already know what is being talked about.

Yes, often it does.

In actual usage:

• 不妊手術 often leans toward spaying / sterilization
• 去勢手術 often refers to neutering / castration
• 不妊去勢手術 can function as a broad term covering both

So this combined expression is especially common when speaking generally about reproductive-prevention surgery for pets.

It is pronounced:

ふにんきょせいしゅじゅつ が おわった。

Romanization:

funin kyosei shujutsu ga owatta

A few pronunciation notes:

• 去勢 = きょせい
• 手術 = しゅじゅつ, which many learners find tricky because of the repeated ju sounds
• 終わった = おわった, with the small っ creating a brief pause before ta

Not directly. This sentence most naturally means:

• The surgery ended
• The surgery is over

If you want to clearly say someone finished the surgery, Japanese would usually use a transitive structure, such as:

• 獣医が手術を終えた。 = The veterinarian finished the surgery.

So your original sentence is less about the doer and more about the completed state of the surgery.

Yes, slightly.

• 終わった focuses on the completion: ended / finished
• 終わっている focuses more on the resulting state: has ended / is finished

So:

• 不妊去勢手術が終わった。 = The spay/neuter surgery ended / is over.
• 不妊去勢手術が終わっている。 = The spay/neuter surgery has already finished / is in a finished state.

In many situations the English translation may look similar, but the Japanese nuance is a little different.