Learn Hardcore Spanish (Latin America): Reprobé solo una materia el año pasado porque no estudié con un plan claro. - I failed only one subject last year because I didn’t study with a clear plan.

You normally do not use the accent: write solo for both meanings.
The accent sólo is now optional and only recommended if there is real ambiguity, which is rare.

In this sentence, solo clearly means only, not alone, and there’s no ambiguity, so solo (no accent) is correct and standard:

Reprobé solo una materia = I failed only one subject.

You could also say solamente or nada más que for emphasis:

Reprobé solamente una materia.

Is there a difference between solo una materia and una sola materia?

Both are correct and both can mean “only one subject.” The difference is nuance:

Solo una materia
Neutral: only one subject. Focus is on “only” modifying “one subject”.
Una sola materia
Slightly stronger emphasis on just one and no more than that. It can sound a bit more emotional or emphatic in context:
- ¡Por suerte reprobé una sola materia!
  Luckily I failed just one subject!

In many contexts they’re interchangeable, but una sola materia can sound more emphatic.

What does materia mean here? Is it “subject,” “class,” or something else?

In Latin American Spanish, materia in a school/university context means roughly school subject or course:

materia de matemáticas = math class / math subject
inscribirse en cinco materias = to enroll in five classes

Other common words:

asignatura – also subject, more formal/neutral.
clase – often refers to the lesson or meeting, or the group of students.
curso – can mean course, sometimes the whole year-level or a specific course.

So Reprobé solo una materia is best understood as “I failed only one subject (or class).”

Why is the past tense reprobé / no estudié used (preterite), and not reprobaba / no estudiaba (imperfect)?

Spanish has two main simple pasts:

Preterite (reprobé, estudié) – completed actions, seen as whole events.
Imperfect (reprobaba, estudiaba) – ongoing, repeated, or background actions.

Here, the speaker talks about:

A specific, completed event: failing one subject last year.
A specific, completed behavior: not studying with a clear plan (as a defined factor in that failure).

So:

Reprobé solo una materia el año pasado…
= I failed only one subject last year… (one completed result)
…porque no estudié con un plan claro.
= …because I didn’t study with a clear plan. (the way they studied in that particular period, seen as a whole)

If you said:

…porque no estudiaba con un plan claro,
it would paint the lack of a clear plan more as a habitual / ongoing background situation during that time, which can also be possible but feels a bit more descriptive and less “event-like.” The original is the most natural choice.

Why is it el año pasado and not en el año pasado or another form?

El año pasado is a fixed time expression meaning “last year.” You normally don’t add en:

El año pasado reprobé solo una materia. ✅
En el año pasado reprobé… ❌ (sounds unnatural in this sense)

Compare:

El mes pasado – last month
La semana pasada – last week

You could also move the phrase:

El año pasado reprobé solo una materia.
Reprobé solo una materia el año pasado.

Both are correct; it’s just a matter of emphasis and style.

What’s the difference between porque, por qué, porqué, and por que? Why is it porque here?

The four forms have different uses:

porque (one word, no accent) – “because”
- Introduces a reason:
- Reprobé solo una materia porque no estudié con un plan claro.
  I failed only one subject because I didn’t study with a clear plan.
por qué (two words, accent on qué) – “why” (in questions)
- ¿Por qué reprobaste la materia?
  Why did you fail the subject?
el porqué (one word, with article, accent) – “the reason”
- No entiendo el porqué de tu decisión.
  I don’t understand the reason for your decision.
por que (two words, no accent) – rarer; appears after certain prepositions/verbs, often equivalent to por el que / por la que, etc.
- Esta es la razón por que / por la que vine.
  This is the reason (for which) I came.

In your sentence, it clearly means “because”, so porque is the correct form.

Why is it estudié con un plan claro and not just estudié un plan claro?

The preposition con here means “with” and indicates the manner or resources used:

estudiar con un plan claro = to study with a clear plan
(You are using a plan as a tool/structure.)

If you say:

estudié un plan claro,
that means I studied a clear plan (the plan is the object of your studying), which is not the intended meaning.

So:

No estudié con un plan claro = I didn’t study with a clear plan (in place).

Why is it plan claro and not claro plan? Where do adjectives normally go?

In Spanish, adjectives usually follow the noun:

plan claro = clear plan
materia difícil = difficult subject
año pasado = last year

So un plan claro is the normal, neutral order.

Some adjectives can come before the noun, but that often changes the nuance (more subjective, emotional, or idiomatic):

un claro ejemplo – a clear (good) example (common fixed phrase)
una gran idea – a great idea (not just “big”)

Claro plan is not natural Spanish here. You should say un plan claro.

Can I change the word order and still be correct? For example, can I start with the reason?

Yes, Spanish allows some flexibility with word order, especially with clauses like porque…. Some natural variations:

Reprobé solo una materia el año pasado porque no estudié con un plan claro.
El año pasado reprobé solo una materia porque no estudié con un plan claro.
Porque no estudié con un plan claro, reprobé solo una materia el año pasado.

All are grammatically correct. Starting with porque (Because I didn’t study with a clear plan…) is also acceptable in Spanish, unlike the more formal rule in English that sometimes avoids starting sentences with Because.

How should I pronounce reprobé, estudié, and año? What are the accents doing?

reprobé – reh-pro-BÉ
estudié – es-tu-di-É
- The written accent (´) shows that the stress falls on the last syllable, even though the word ends in a vowel (which normally would be stressed on the second-to-last syllable).
año – Á-nyo
- ñ is a separate letter, pronounced like the ny in canyon: /ɲ/.
- Be careful: año (year) vs ano (anus). The ñ is important both in writing and pronunciation.

The accents in reprobé and estudié are required for correct stress in these past-tense forms.