Me hice daño en un dedo y me rompí una uña al cerrar la puerta del coche.

Hacerse daño is a very common way to say to hurt oneself / to get hurt in everyday Spanish.

• Me hice daño = I hurt myself / I got hurt
• Me dañé exists, but dañar often sounds more formal, more technical, or more like causing damage in a broader sense.

So in normal spoken Spanish, especially for minor injuries, me hice daño is much more natural.

In both parts, me shows that the action happened to the speaker.

• me hice daño = I hurt myself
• me rompí una uña = literally I broke myself a nail, meaning I broke a nail

Spanish often uses a reflexive or indirect-object-style structure for things that happen to your body. English usually does not repeat myself or to me in the same way, but Spanish often does.

Because en is used here to mean on or in the area of a body part.

• Me hice daño en un dedo = I hurt one of my fingers / I hurt myself on a finger

Using a here would not sound natural. With injuries or pain, Spanish commonly uses:

• en la mano
• en el brazo
• en un dedo

So en is the normal preposition.

Spanish often uses the article or an indefinite article with body parts instead of a possessive, especially when it is already obvious whose body part it is.

Here, because me already tells us it happened to me, Spanish does not need mi.

• me rompí una uña sounds natural
• me rompí mi uña would usually sound unnecessary or awkward

Also, un dedo and una uña help express one finger and a nail, not a specific previously identified one.

Because the nouns have different grammatical genders:

• dedo is masculine → un dedo
• uña is feminine → una uña

This is something you just have to learn with the noun.

A useful pair to remember:

• el dedo
• la uña

The preterite is used because the sentence describes completed actions in the past:

• me hice daño
• me rompí una uña

These are single events that happened at a specific moment, not ongoing situations. In English, this matches the simple past:

• I hurt myself
• I broke a nail

If the sentence were describing a background situation or a habitual action, Spanish might use the imperfect instead, but not here.

Al + infinitive is a very common Spanish structure meaning:

• when ...
• upon ...
• while ...
  depending on context.

So:

• al cerrar la puerta del coche = when closing the car door / as I closed the car door

It literally comes from a + el, but as a structure it functions like when doing something.

Other examples:

• Al entrar, saludó a todos. = When he came in, he greeted everyone.
• Al salir, apaga la luz. = When leaving, turn off the light.

Because de + el contracts to del.

So:

• de el coche → del coche

This contraction is mandatory in normal Spanish.

The two main contractions to remember are:

• a + el = al
• de + el = del

That is why the sentence has both:

• al cerrar
• del coche

Both are possible, but they are slightly different in feel.

• Me rompí una uña = I broke a nail
  • This can sound a bit more direct, as if it happened in the course of what I did.
• Se me rompió una uña = A nail broke on me / I ended up breaking a nail
  • This often sounds a little more accidental or less intentional.

In your sentence, me rompí una uña is perfectly natural because it clearly happened as a result of closing the car door.

Yes, absolutely. Spanish allows that word order very naturally.

For example:

• Al cerrar la puerta del coche, me hice daño en un dedo y me rompí una uña.

This version puts the time/circumstance first: when closing the car door...

The original sentence is also natural. Spanish is often flexible with word order as long as the meaning stays clear.

In Spain, coche is the normal everyday word for car.

• Spain: coche
• Many parts of Latin America: carro or auto

Since this sentence is in Spanish from Spain, coche is the expected choice.

The ñ is a separate Spanish letter. It sounds roughly like ny in canyon.

So uña is approximately:

• OO-nya

Also notice that the u is pronounced here. The word has two syllables:

• u-ña

This is different from plain n:

• una = OO-na
• uña = OO-nya

That difference matters.

In me hice daño, daño is a noun meaning harm or injury.

Literally, the structure is something like:

• I did harm to myself

But in natural English, we say:

• I hurt myself

So even though English uses the verb hurt, Spanish often uses:

• hacerse daño

This is one of those cases where translating word for word is less helpful than learning the whole expression.