El dobladillo de mis pantalones se soltó y ahora necesito arreglarlo.

In Spanish, pantalones is normally used in the plural, just like trousers in British English or pants in English generally.

So you say:

• mis pantalones = my trousers / my pants
• los pantalones = the trousers / the pants

The singular pantalón usually means one trouser leg in some contexts, or it may appear in fixed expressions, but for the item of clothing as a whole, learners should normally use pantalones.

Dobladillo means hem: the folded and sewn edge at the bottom of a piece of clothing.

In this sentence, el dobladillo de mis pantalones means the hem of my trousers/pants.

It is a very normal word if you are talking about clothing repairs, sewing, or tailoring.

Spanish often prefers noun + de + owner when it wants to be precise about what something belongs to.

So:

• el dobladillo de mis pantalones = the hem of my trousers

This sounds natural because the hem is a part of the trousers, not really a separate possession in the same way as mi libro or mi móvil.

You could theoretically understand mi dobladillo, but it would sound incomplete or odd here, because a dobladillo is usually the hem of something, so Spanish normally says what item it belongs to.

Here soltarse is being used as a pronominal verb.

• soltar usually means to release, to let go of, or to loosen something
• soltarse can mean to come loose, to come undone, or to loosen itself

So:

• solté el botón = I loosened / released the button
• el botón se soltó = the button came loose

In your sentence:

• El dobladillo ... se soltó = The hem came loose

The se does not really mean the hem is doing the action on purpose. It is part of the verbal expression meaning that something became loose.

Not in the literal English sense of it loosened itself.

Grammatically, it uses the reflexive/pronominal form, but in meaning it is better understood as came loose or came undone. This is very common in Spanish.

So for learners, it is best to remember:

• soltar = to loosen/release something
• soltarse = to come loose

Se soltó is the preterite, which is used for a completed event.

Here, the sentence refers to a specific thing that happened:

• the hem came loose

That is why se soltó fits well.

If you said se soltaba, it would usually suggest an ongoing, repeated, or background situation, such as:

• El dobladillo se soltaba constantemente = The hem kept coming loose

So in your sentence, se soltó is the natural choice because it describes one completed event.

Yes. In Spain, se ha soltado is also very natural, especially if the speaker feels the event is recent and relevant now.

So both can work:

• se soltó = it came loose
• se ha soltado = it has come loose

In Spain Spanish, the present perfect is often used for recent events connected to the present. The preterite is also possible, depending on context and region.

Because necesitar is commonly followed by an infinitive when you mean to need to do something.

So:

• necesito arreglarlo = I need to fix it
• necesito coserlo = I need to sew it
• necesito lavarlos = I need to wash them

This is the normal Spanish pattern:

• necesitar + infinitive

Lo refers to el dobladillo.

Since dobladillo is:

• singular
• masculine

the direct object pronoun is lo.

So:

• arreglar el dobladillo = to fix the hem
• arreglarlo = to fix it

It does not refer to pantalones, because that word is plural, and then the pronoun would be los.

Because the thing being fixed is the hem, not the trousers as a whole.

The main noun being referred to is:

• el dobladillo → masculine singular → lo

So:

• necesito arreglarlo = I need to fix it
  (it = the hem)

If you wanted to say you need to fix the trousers themselves, then you could use los:

• Necesito arreglarlos = I need to fix them
  (them = the trousers)

Yes, depending on what you mean.

• arreglarlo = to fix it
  Very general and natural.
• coserlo = to sew it
  More specific: you are saying the repair involves sewing.
• repararlo = to repair it
  Correct, but a bit more formal or technical in many everyday contexts.

For clothing, arreglarlo is a very natural everyday choice.

Yes. In Spain, arreglar is very commonly used for fixing many things, including clothing.

Examples:

• Tengo que arreglar estos pantalones = I have to fix these trousers
• Me arreglaron la cremallera = They fixed the zip
• Voy a arreglar el dobladillo = I’m going to fix the hem

For clothes, you may also hear more specific verbs like coser, remendar, or ajustar, but arreglar is an excellent general verb.

Yes, but the meaning changes slightly.

• se soltó = it came loose
  This focuses on the event.
• está suelto = it is loose
  This focuses on the current state.

So:

• El dobladillo de mis pantalones se soltó = The hem of my trousers came loose
• El dobladillo de mis pantalones está suelto = The hem of my trousers is loose

Both are useful, but they are not exactly the same.

Yes, very natural.

That version adds me, which often gives a nuance that this happened to me or on me, often unintentionally.

• Se me soltó el dobladillo de los pantalones = The hem of my trousers came loose on me

This structure is very common in Spanish when something happens accidentally or unexpectedly.

Compared with your original sentence:

• El dobladillo de mis pantalones se soltó = neutral statement
• Se me soltó el dobladillo de los pantalones = slightly more personal, often more idiomatic in conversation

Because the sentence is about what specifically came loose: the hem.

Spanish chooses the most relevant thing as the subject:

• El dobladillo de mis pantalones se soltó
  The hem of my trousers came loose

If you started with Mis pantalones, you would usually need a different structure, for example:

• Mis pantalones tienen el dobladillo suelto = My trousers have a loose hem

So the original sentence highlights the exact part that needs attention.

In standard Spain Spanish, a rough guide is:

• do-bla-DEE-yo or do-bla-DEE-lyo, depending on accent

A few pronunciation notes:

• The stress is on -di-: doblaDILlo
• ll in much of modern Spain is pronounced like y in English yes
• The d between vowels is softer than in English

You do not need to pronounce it perfectly to be understood, but the stress on -dil- is important.