Mi amiga se puso a reír porque su hermano, igual de despistado que ella, olvidó el paraguas otra vez.

Ponerse a + infinitive is a very common structure meaning to start doing something, often with a sense of the action beginning suddenly.

So:

• se puso a reír = she started laughing / she burst out laughing
• rió = she laughed
• empezó a reír = she began to laugh

All are possible in different contexts, but se puso a reír often sounds a bit more vivid or immediate, as if the laughter began at that moment.

Here, se puso comes from the verb ponerse. In this pattern, ponerse a + infinitive, the verb is pronominal, so the se is part of the construction.

You can think of it as a fixed expression:

• me puse a leer = I started reading
• se puso a llorar = he/she started crying
• nos pusimos a hablar = we started talking

So the se is not optional here.

After ponerse a, Spanish normally uses the plain infinitive:

• ponerse a reír
• ponerse a llorar
• ponerse a pensar

Even though reírse is also a common verb meaning to laugh, the standard expression here is ponerse a reír, not ponerse a reírse.

So this is something learners usually just have to remember as part of the idiom.

Despistado means something like absent-minded, scatterbrained, forgetful, or not paying attention.

In Spain, it is a very common everyday word. In this sentence, it suggests that the brother is the kind of person who forgets things easily or is not very organized.

Examples:

• Soy muy despistado. = I’m very absent-minded.
• Siempre se deja las llaves. Es muy despistado. = He always forgets his keys. He’s very absent-minded.

This is a comparison structure:

• igual de + adjective + que = just as + adjective + as

So:

• igual de despistado que ella = just as absent-minded as she is

Other examples:

• Es igual de alto que su padre. = He is just as tall as his father.
• Soy igual de tímida que mi hermana. = I’m just as shy as my sister.

This structure is very useful and very common.

Because despistado describes su hermano, not ella.

In the phrase:

• su hermano, igual de despistado que ella, ...

the adjective agrees with hermano, which is masculine singular, so it must be despistado.

If the subject were feminine, then it would change:

• su hermana, igual de despistada que él, ...

So the comparison is with ella, but the adjective still agrees with the person being described.

The commas show that this part is extra descriptive information about su hermano.

The core sentence is:

• Mi amiga se puso a reír porque su hermano olvidó el paraguas otra vez.

The part between commas adds a comment:

• igual de despistado que ella

That inserted phrase is called a parenthetical or appositive-type comment. It gives more detail but is not essential to the basic structure of the sentence.

Spanish often drops subject pronouns, but here ella is not being used as a subject pronoun in the usual sense. It appears after que in a comparison:

• igual de despistado que ella

This means just as absent-minded as she is.

In this structure, Spanish commonly uses the stressed pronoun:

• que ella
• que él
• que yo
• que tú

So ella is needed here to complete the comparison clearly.

Olvidó is the preterite, which is used for a completed action at a specific moment:

• olvidó el paraguas = he forgot the umbrella

This is seen as a single completed event.

If you used olvidaba, it would usually suggest a repeated, habitual, or background action:

• Siempre olvidaba el paraguas. = He always used to forget the umbrella.

Because the sentence is referring to one specific occurrence, olvidó is the natural choice.

Otra vez means again.

Here it tells us that this is not the first time the brother has forgotten the umbrella. It adds a humorous or slightly frustrated tone:

• olvidó el paraguas otra vez = he forgot the umbrella again

It helps explain why the friend started laughing: this seems to be something he does repeatedly.

Yes, it could move, but the end position is very natural and common.

The sentence says:

• olvidó el paraguas otra vez

This is the most neutral order.

You could also hear:

• otra vez olvidó el paraguas
• olvidó otra vez el paraguas

But the original version sounds very natural and keeps the focus on the repeated mistake at the end.

In Spanish, articles are used more often than in English. So where English might say he forgot his umbrella or simply forgot the umbrella, Spanish often uses the definite article:

• olvidó el paraguas

Even if it is understood to be his umbrella, Spanish does not always need a possessive adjective if the context already makes it clear.

This is very common:

• Me duele la cabeza. = My head hurts.
• Se quitó la chaqueta. = He took off his jacket.

Here porque simply introduces the reason:

• Mi amiga se puso a reír porque... = My friend started laughing because...

It explains why she started laughing.

It is worth noticing that porque is one word. Learners often confuse:

• porque = because
• por qué = why

So:

• Se rió porque olvidó el paraguas. = She laughed because he forgot the umbrella.
• ¿Por qué se rió? = Why did she laugh?

The sentence breaks down like this:

• Mi amiga = subject
• se puso a reír = main verb phrase
• porque = introduces the reason
• su hermano = subject of the subordinate clause
• igual de despistado que ella = extra descriptive phrase about su hermano
• olvidó el paraguas otra vez = what he did

So the skeleton is:

• Mi amiga se puso a reír porque su hermano olvidó el paraguas otra vez.

And the middle phrase adds characterization:

• igual de despistado que ella

This is a good example of how Spanish can insert descriptive information inside a sentence without changing the main grammar.