Guardo una horquilla y una diadema en el bolso por si se le suelta la coleta.

Guardar here means to keep / to store / to have put away. The speaker is saying they keep those items in the bag.

• Guardo una horquilla... en el bolso = I keep a hairpin... in my/the bag
• Llevo una horquilla... en el bolso would mean more like I’m carrying a hairpin... in my bag

In many situations, both could work, but guardo emphasizes that the items are kept there for that purpose.

In Peninsular Spanish:

• horquilla = bobby pin / hairpin
• diadema = headband / hairband
• bolso = handbag / purse
• coleta = ponytail

A useful note: diadema is usually a headband, not a general elastic band for tying hair.

Because Spanish often repeats the article before each noun:

• una horquilla y una diadema

This is very natural and standard. English can sometimes sound fine with just one article, but Spanish often prefers to repeat it when talking about separate items.

You could think of it as:

• a hairpin and a headband

Repeating una makes it clear these are two distinct objects.

Spanish often leaves out possessives when the owner is obvious from context.

So:

• en el bolso = in the bag
• but in context it usually means in my bag

This is very common in Spanish. The language does not always use mi, tu, su, etc. when the listener can already work out whose thing it is.

Por si means in case.

So:

• por si se le suelta la coleta = in case her ponytail comes loose

It introduces a possible situation that the speaker wants to be prepared for.

A very important grammar point: after por si, Spanish normally uses the indicative, not the subjunctive.

So this is correct:

• por si se le suelta la coleta

Not normally:

• por si se le suelte la coleta

This is one of the most useful parts of the sentence.

se suelta

The verb is soltarse, which here means to come loose / come undone.

So:

• la coleta se suelta = the ponytail comes loose

le

Le means to her here. It shows the person affected by what happens.

So:

• se le suelta la coleta = literally something like
  the ponytail comes loose on her / to her

Natural English:

• her ponytail comes loose

This structure is very common in Spanish when something happens to someone in a way that affects them.

Because Spanish often uses:

• an indirect object pronoun (me, te, le, nos...)
• plus the definite article (el, la, los, las)

instead of a possessive.

So instead of:

• su coleta

Spanish often prefers:

• le ... la coleta

This is similar to:

• Me duele la cabeza = My head hurts
  • literally: The head hurts to me

So:

• se le suelta la coleta is more idiomatic than se suelta su coleta

No. It is not a passive sentence.

Here, la coleta is the thing that comes loose, and le marks the person affected.

So this is an active idea with an intransitive/pronominal verb, not a passive one.

Compare:

• La coleta se le suelta = Her ponytail comes loose
• Passive would be something more like La coleta es soltada..., which sounds unnatural here.

Yes, you could, but it changes the feel slightly.

• se le suelta la coleta sounds more natural when you want to show that it happens to her
• se suelta la coleta just says the ponytail comes loose

The version with le is often preferred because it adds the idea of the affected person, which is very common in everyday Spanish.

The sentence does not name the person explicitly, but le tells us it is to her / for her.

So the speaker is probably talking about:

• a daughter
• a child
• a friend
• someone already understood from context

If it were the speaker’s own ponytail, you would say:

• por si se me suelta la coleta = in case my ponytail comes loose

If it were your ponytail:

• por si se te suelta la coleta

Yes. Spanish allows some flexibility.

For example, these are possible:

• Guardo una horquilla y una diadema en el bolso por si se le suelta la coleta.
• Por si se le suelta la coleta, guardo una horquilla y una diadema en el bolso.

The original order sounds very natural because it gives:

1. the action first
2. then the reason

That is a common and smooth way to structure the sentence.

In Spain, horquilla is a very normal word for a bobby pin / hairpin.

But learners should know that hair accessories vary a lot by region, and some words do not match perfectly across all Spanish-speaking countries.

For Spain, the sentence sounds natural:

• horquilla
• diadema
• coleta
• bolso

So if you are learning Spanish from Spain, these are good words to know.

Yes. A more literal unpacking would be:

• por si = in case
• se suelta = comes loose
• le = to her
• la coleta = the ponytail

So literally:

• in case the ponytail comes loose on her

That sounds odd in English, but it helps explain why Spanish uses:

• le
• la coleta instead of simply her ponytail

Natural English then becomes:

• in case her ponytail comes loose