Soy tan despistado que ayer fui a clase sin el cuaderno, aunque había preparado la mochila por la noche.

Because ser is used for a more general characteristic, while estar usually describes a temporary state.

• Soy despistado = I’m an absent-minded / forgetful kind of person.
• Estoy despistado = I’m feeling confused or absent-minded right now.

In this sentence, the speaker is describing their usual tendency, not just a one-time mood, so soy fits better.

Tan ... que means so ... that.

The pattern is:

• tan + adjective/adverb + que
• tanto/a/os/as + noun + que

Here:

• tan despistado que... = so absent-minded that...

So the sentence is setting up a cause-and-result idea:

• I’m so absent-minded that yesterday I went to class without the notebook...

Here, despistado means something like:

• absent-minded
• forgetful
• scatterbrained
• not very attentive

It does not usually mean simply distracted in the English sense of being unable to focus because of noise or interruptions, although there is some overlap.

Also, it changes with gender:

• despistado = masculine
• despistada = feminine

So a woman would say:

• Soy tan despistada que...

Fui is the preterite of ir, and it is used for a completed action in the past:

• ayer fui a clase = yesterday I went to class

Why not the others?

• iba would suggest an ongoing past action, a habitual action, or background description:
  • Ayer iba a clase cuando... = Yesterday I was on my way to class when...
• he ido is the present perfect. In Spain, it is common for recent past actions, but with ayer, standard usage strongly prefers the preterite:
  • ayer fui, not usually ayer he ido

So fui is the natural choice here.

Because ir a clase is a very common fixed expression meaning to go to class or to attend class in a general sense.

Compare:

• fui a clase = I went to class
• fui a la clase de español = I went to the Spanish class / the Spanish lesson

So:

• a clase = class in general, as an activity or institution
• a la clase = a specific class

Spanish often uses the definite article where English would use a possessive, especially when it is obvious whose thing it is.

So:

• sin el cuaderno = without the notebook

This sounds natural if everyone understands it is your notebook.

You could also say:

• sin mi cuaderno

That would add a bit more emphasis to the fact that it was my notebook specifically.

Also compare:

• sin cuaderno = without a notebook / notebook-less in a more general sense
• sin el cuaderno = without the notebook that was expected or relevant

Because había preparado is the pluperfect (past perfect), and it shows that this action happened before another past action.

Timeline:

1. First, the speaker packed the backpack at night.
2. Later, the next day, they went to class without the notebook.

So:

• había preparado la mochila = I had packed/prepared the backpack

This makes the sequence very clear.

If you said preparé la mochila, it would simply mean I packed the backpack, but it would not show the earlier-before-the-other-past-event relationship as clearly.

Aunque means although / even though.

It introduces a contrast:

• The speaker had packed the backpack the night before
• but still forgot the notebook

So the idea is:

• Even though I had prepared my backpack the night before, I still went to class without the notebook.

It highlights the irony or contradiction.

In Spain, por la noche is the normal way to say at night or in the evening/at night, depending on context.

So:

• había preparado la mochila por la noche = had packed the backpack that night / the night before

En la noche is much less common in Spain in everyday speech. It can sound more literary, regional, or like during the night in some varieties.

For a learner of Spanish from Spain, por la noche is the safest and most natural option here.

Because Spanish often leaves out subject pronouns when they are not needed.

The verb already tells you the subject:

• soy = I am
• fui = I went
• había preparado = I had prepared

So yo is unnecessary unless you want emphasis or contrast.

Compare:

• Soy tan despistado... = neutral
• Yo soy tan despistado... = more emphatic, maybe contrasting with someone else

Yes, Spanish word order is fairly flexible, although some versions sound more natural than others.

For example, you could say:

• Aunque había preparado la mochila por la noche, ayer fui a clase sin el cuaderno porque soy tan despistado.

That is grammatical, but it changes the rhythm and emphasis.

The original version:

• Soy tan despistado que...

puts the speaker’s personality trait first and then gives the result. That makes the sentence feel very natural and expressive.

So yes, the order can change, but the original structure is good because it neatly sets up cause → consequence.

Not in the same structure.

• muy despistado = very absent-minded
• tan despistado que... = so absent-minded that...

If you want the result clause with que, you normally need tan:

• Soy tan despistado que olvidé el cuaderno.

If you only want to describe the person without giving a consequence, then muy works:

• Soy muy despistado.

So:

• muy = very
• tan ... que = so ... that

Yes. In Spain, mochila is a very common word for backpack or a school bag worn on the back.

So:

• preparar la mochila = to pack/get your backpack ready

Depending on context, other words exist, but mochila is completely natural and standard in Spain, especially for a student.