Su oído mejoró.

Su is ambiguous in Spanish. It can mean:

• his
• her
• your (formal)
• their

So Su oído mejoró could mean His hearing improved, Her hearing improved, Your hearing improved (formal), or Their hearing improved, depending on context.

In Spain, if you mean your for more than one person informally, you would usually use vuestro, not su.

Because su agrees with the thing possessed, not with the owner.

Here, oído is singular, so you use su:

• su oído = his/her/your/their ear/hearing

If the noun were plural, you would use sus:

• sus oídos = his/her/your/their ears

This is a very common point of confusion for English speakers.

It can mean either, depending on context.

• ear as a body part
• hearing as the sense or ability to hear

In this sentence, it most naturally means hearing, because hearing improved sounds more natural than ear improved in English.

Also, Spanish often uses oreja for the outer ear, while oído can refer more to hearing or to the ear in a broader or more internal sense.

The accent mark shows that the stress falls on í:

• o-Í-do

Without the accent, Spanish pronunciation rules would suggest a different syllable pattern.

The accent is also there because o and í are pronounced in separate syllables here, not as one diphthong. This is called a hiatus.

Mejoró is the preterite tense, third person singular, of the verb mejorar.

So it means:

• he/she/it improved
• got better

In this sentence, the subject is su oído, so mejoró means improved or got better.

Because mejoró presents the improvement as a completed change.

• mejoró = improved / got better
• mejoraba = was improving / used to improve

So:

• Su oído mejoró = his/her hearing improved
• Su oído mejoraba = his/her hearing was improving

The preterite is the normal choice when you want to say that a change happened and was completed.

No. Here mejorar is used non-reflexively.

• Su oído mejoró = His/her hearing improved

Spanish often allows mejorar to work this way, with the thing itself as the subject.

You may also see reflexive forms like mejorarse, especially with people or health conditions, but in this sentence the non-reflexive form is completely normal.

Yes. Mejoró su oído is also grammatical.

Both mean the same thing, but the emphasis is slightly different:

• Su oído mejoró = more neutral, starting with the subject
• Mejoró su oído = slightly more focused on the action or result

Spanish word order is often more flexible than English word order.

Roughly:

• meh-ho-RO

A few details:

• the j is pronounced like a strong throaty sound, not like the English j
• the final syllable is stressed: mejorÓ

So the full sentence is pronounced approximately:

• su o-Í-do meh-ho-RO

Yes. Oído is related to oír, which means to hear.

In fact, oído is also the past participle of oír:

• He oído eso = I have heard that

But in Su oído mejoró, oído is a noun, not a verb form. Here it means hearing or ear.