Después de haberse secado, mi hermana guardó la toalla y se puso el pijama.

Because after después de, Spanish commonly uses:

• después de + infinitive when the subject is the same
• después de que + conjugated verb when you want a full clause

So here, since mi hermana is the one doing both actions, después de haberse secado is natural.

Después de se secó is not possible, because a conjugated verb cannot go directly after de like that.

A full-clause alternative would be después de que se secó, but with the same subject, Spanish often prefers the infinitive structure.

It is a perfect infinitive (also called a compound infinitive).

It is made of:

• haber in the infinitive
• the reflexive pronoun se
• the past participle secado

So:

• haber secado = to have dried
• haberse secado = to have dried oneself / to have got dry

This form shows that the drying happened before the later actions: guardó and se puso.

Because Spanish allows object and reflexive pronouns to attach to:

• infinitives
• gerunds
• affirmative commands

Since haber is an infinitive here, the reflexive pronoun attaches to it: haberse.

That is why you get:

• haberse secado
• not se haber secado

The same thing happens in forms like ponerse, lavarse, irse, etc.

Yes. Después de secarse is also correct and very natural.

The difference is mainly one of emphasis:

• después de secarse = after drying off / after getting dry
• después de haberse secado = after having dried off

The version with haber makes the earlier completion a little more explicit. In everyday speech, many speakers would simply say después de secarse.

Because secar and secarse mean different things:

• secar = to dry something
  
  • Secó la toalla = She dried the towel
• secarse = to dry oneself / to get dry
  
  • Se secó = She dried herself / She got dry

In this sentence, the sister is the one becoming dry, so the reflexive form is used.

They are in the preterite because the sentence narrates completed actions in sequence.

• guardó la toalla = she put away the towel
• se puso el pijama = she put on her pyjamas

This is a typical use of the Spanish preterite: a chain of finished events.

If the imperfect were used instead, it would suggest habit, background description, or an ongoing action, which is not the point here.

Because Spanish normally uses the verb ponerse for clothing:

• ponerse una chaqueta
• ponerse los zapatos
• ponerse el pijama

Also, Spanish often uses the definite article instead of a possessive when it is obvious whose item it is.

So se puso el pijama sounds very natural: the sister put on her pyjamas, even though her is not explicitly stated.

Puso su pijama is possible in some contexts, but it sounds less natural here and may feel more emphatic or contrastive.

In Spain, pijama is normally a singular masculine noun: el pijama.

Even though English often uses pajamas/pyjamas as a plural-looking word, Spanish usually treats the whole item as one thing:

• el pijama
• un pijama nuevo

So se puso el pijama is the standard way to say it in Spain.

No. In Spanish, guardar very often means to put away, to store, or to put back in its place.

So here guardó la toalla means something like:

• she put the towel away
• she put the towel back
• she stored the towel

It does not mean she stood there protecting the towel.

This is a very common false friend for English speakers.

Because Spanish can omit subjects, but it does not have to.

The noun mi hermana is included here to identify who is doing the actions. Once that is clear, the later verbs do not need the subject repeated.

So this is completely natural:

• Mi hermana guardó la toalla y se puso el pijama

If the context already made it obvious, Spanish could also omit the noun:

• Después de haberse secado, guardó la toalla y se puso el pijama

Se puso is from the pronominal verb ponerse, which often means to put on when talking about clothes.

So:

• poner = to put, place
• ponerse = to put on oneself

The se in se puso belongs to ponerse, while the se in haberse secado belongs to secarse. They are both reflexive pronouns, but each one is tied to its own verb.

So the sentence contains two different reflexive verbs:

• secarse
• ponerse

Yes. Spanish allows some flexibility.

For example, you could also say:

Mi hermana guardó la toalla y se puso el pijama después de haberse secado.

This means the same thing. Starting with Después de haberse secado simply puts the time sequence first and makes the sentence feel a bit more structured or narrative.

The comma after the opening phrase is natural and helps readability.