Cuando me duele la oreja, me la toco sin darme cuenta.

Because the sentence is talking about a habitual or general situation: whenever this happens, this is what I do.

So:

Cuando me duele la oreja, me la toco sin darme cuenta.
= Whenever my ear hurts, I touch it without realizing it.

If you were talking about a specific future situation, Spanish often uses the present subjunctive after cuando:

Cuando me duela la oreja, iré al médico.
= When my ear hurts, I’ll go to the doctor.

So:

• cuando + present indicative = habitual/repeated situations
• cuando + present subjunctive = future events

Literally, it works more like it hurts to me than I hurt.

With doler, Spanish structures things differently from English:

• la oreja is the thing that hurts
• me means to me

So:

Me duele la oreja
literally: The ear hurts to me
natural English: My ear hurts

This is the same pattern as:

• Me duele la cabeza = My head hurts
• Me duelen los pies = My feet hurt

Because with body parts, Spanish usually prefers:

• a definite article: el, la, los, las
• plus a pronoun showing whose body it is

So instead of my ear hurts, Spanish often says:

Me duele la oreja
literally: The ear hurts to me

The me already tells you whose ear it is, so mi is usually unnecessary.

This is very common in Spanish:

• Me lavo las manos = I wash my hands
• Me duele la espalda = My back hurts
• Se rompió la pierna = He/She broke his/her leg

Because the subject is la oreja, which is singular.

With doler, the verb agrees with the thing that hurts, not with the person.

So:

• Me duele la oreja = My ear hurts
• Me duelen las orejas = My ears hurt

Compare:

• Me duele el brazo = My arm hurts
• Me duelen los brazos = My arms hurt

They each do a different job:

• la = it, referring to la oreja
• me = shows that I am doing the action to myself / on my own body

So:

me la toco
= I touch it on myself
natural English: I touch it

Spanish often uses a reflexive pronoun with body parts, where English just uses a possessive:

• Me lavo las manos = I wash my hands
• Me toco la cara = I touch my face

Here, since la oreja has already been mentioned, Spanish replaces it with la:

• me toco la oreja
• me la toco

Both are natural, but me la toco avoids repeating la oreja.

Yes, absolutely.

Both are correct:

• Me toco la oreja = I touch my ear
• Me la toco = I touch it

The difference is mainly that la is a pronoun replacing la oreja, which was already mentioned earlier in the sentence.

So this sentence avoids repetition:

Cuando me duele la oreja, me la toco...

If you repeated the noun, it would still be fine:

Cuando me duele la oreja, me toco la oreja...

But that sounds a bit more repetitive.

Because with body parts, Spanish normally includes the reflexive pronoun to show the body part belongs to the subject.

So:

• me la toco = I touch my own ear
• la toco = I touch it

Without me, the sentence is more neutral and does not strongly mark that it is my own ear. In this context, since it is your body part, me la toco is the natural Spanish choice.

It means without realizing it or without noticing.

This comes from the expression:

darse cuenta (de algo)
= to realize / to notice / to become aware (of something)

So:

• Me doy cuenta = I realize / I notice
• No me doy cuenta = I don’t realize
• Sin darme cuenta = without realizing it

In your sentence:

Cuando me duele la oreja, me la toco sin darme cuenta.
= When my ear hurts, I touch it without realizing it.

Because in Spanish, pronouns are often attached to an infinitive.

So:

• dar + me = darme
• tocar + la = tocarla
• lavar + se = lavarse

After sin, Spanish commonly uses an infinitive:

sin darme cuenta
= without realizing it

You could think of it as:

• sin
  • darme cuenta

This attachment is normal with:

• infinitives: quiero verlo
• gerunds: está mirándome
• affirmative commands: mírame

Good question. Darse cuenta is very often followed by de when you explicitly say what you realize:

• Me doy cuenta de mi error. = I realize my mistake.
• Se dio cuenta de que era tarde. = He/She realized it was late.

But in sin darme cuenta, the idea is more general and the object is left unstated. Spanish very commonly leaves it like that.

So:

• sin darme cuenta = without realizing it / without noticing

You could expand it in some contexts, but you usually do not need to.

Not exactly.

• oreja usually refers to the outer ear or simply the ear as a visible body part
• oído often refers more to hearing or the inner/auditory sense

Examples:

• Me duele la oreja = My ear hurts
• Tengo buen oído = I have a good ear / good hearing
• Perdió el oído = He/She lost hearing

In your sentence, oreja is the natural word because it is a body part you can physically touch.

The comma separates the time clause from the main clause:

• Cuando me duele la oreja = when my ear hurts
• me la toco sin darme cuenta = I touch it without realizing it

This is very common when the subordinate clause comes first.

So the sentence is easier to read as:

Cuando me duele la oreja,
me la toco sin darme cuenta.

In informal writing, people are not always perfectly consistent with commas, but this punctuation is standard and natural.