Learn Hardcore Spanish (Spain): Mi hermano está enfadado porque olvidé su cumpleaños. - My brother is angry because I forgot his birthday.

Está enfadado = He is angry (right now), a temporary emotional state.
Ser enfadado would suggest that being angry is a permanent trait, which is not how Spanish normally expresses this idea. It would sound strange or wrong in most contexts.

So with emotions like enfadado, triste, contento, nervioso, you almost always use estar:

Mi hermano está enfadado / triste / nervioso / contento.

Why is it enfadado and not enfadada?

Adjectives in Spanish agree in gender and number with the noun they describe.

Mi hermano is masculine singular.
The adjective must therefore also be masculine singular: enfadado.

If it were a sister, you would say:

Mi hermana está enfadada porque olvidé su cumpleaños.

Plural examples:

Mis hermanos están enfadados.
Mis hermanas están enfadadas.

What’s the difference between enfadado, enojado, and cabreado?

All three can mean angry, but there are differences in use and tone.

enfadado
- Very common in Spain.
- Neutral register: fine in both everyday and polite speech.
enojado
- Used a lot in Latin America.
- Also understood in Spain, but sounds more Latin American or formal/standard.
cabreado
- Very colloquial in Spain.
- Stronger and more informal, often closer to really pissed off than just angry.

So in Spain:

Mi hermano está enfadado is the safest, most neutral choice.
Mi hermano está cabreado is common in casual conversation, but more vulgar/colloquial.

Why is there no yo before olvidé?

Spanish usually drops subject pronouns (yo, tú, él, etc.) when the verb ending already shows who the subject is.

olvidé can only be I forgot, so yo is not needed:
- olvidé su cumpleaños = I forgot his birthday.

You only add yo for emphasis or contrast:

Yo olvidé su cumpleaños, no mi hermana.
(I forgot his birthday, not my sister.)

Why is it olvidé and not he olvidado or olvidaba?

olvidé is the simple past (pretérito indefinido) and presents the action as a finished event at a specific point in the past.

olvidé su cumpleaños = I forgot his birthday (on that occasion, it happened and it’s over).

Other options:

he olvidado su cumpleaños (pretérito perfecto)
- In Spain, often used for recent past events that are connected to the present.
- Suggests I have forgotten his birthday (and this is relevant now).
- In many parts of Spain, both olvidé and he olvidado could be heard, depending on context and speaker preference.
olvidaba su cumpleaños (imperfecto)
- Describes a repeated or ongoing past situation:
  - Siempre olvidaba su cumpleaños. = I used to always forget his birthday.

In your sentence, olvidé is a natural choice because it refers to one specific past incident.

Can I also say Me olvidé de su cumpleaños? Is there a difference?

Yes, you can. Both are correct, but the structure is different:

Olvidé su cumpleaños.
- olvidar + direct object
- More direct and often a bit more formal/standard.
Me olvidé de su cumpleaños.
- olvidarse de + object, with a reflexive pronoun (me)
- Very common in everyday speech, especially in Spain and much of Latin America.

They mean practically the same: I forgot his birthday.

What you must not say is:

✗ olvidé de su cumpleaños (wrong: non‑reflexive olvidar cannot take de).

So you have two correct patterns:

olvidar algo
olvidarse de algo

Why is it porque (one word, no accent) and not por qué or porqué?

Spanish distinguishes these forms:

porque (one word, no accent) = because
- Introduces a reason:
  - Está enfadado porque olvidé su cumpleaños.
    He is angry because I forgot his birthday.
por qué (two words, accent on qué) = why (in questions)
- ¿Por qué está enfadado? = Why is he angry?
el porqué (one word, accent, with article) = the reason
- No entiendo el porqué de su enfado. = I don’t understand the reason for his anger.

In your sentence, we are giving a reason, so porque is correct.

How does su cumpleaños work? Could I say el cumpleaños de mi hermano instead?

su is a possessive that can mean his, her, their, your (formal), depending on context.

su cumpleaños here clearly means his birthday, because we just mentioned mi hermano.

You can also say:

el cumpleaños de mi hermano = my brother’s birthday

Both are correct. Differences:

su cumpleaños
- Shorter, very natural.
- Can be ambiguous out of context (his/her/their/your), but the context usually clarifies it.
el cumpleaños de mi hermano
- Longer, but completely unambiguous about whose birthday it is.
- Often used when you want to be very clear.

In this specific sentence, both versions sound natural.

Can the word order change? For example, can I move the porque clause?

Yes, Spanish word order is somewhat flexible. You can say:

Mi hermano está enfadado porque olvidé su cumpleaños. (most common)
Porque olvidé su cumpleaños, mi hermano está enfadado. (more emphasis on the cause)

Both are correct. The version with porque at the end is more typical and neutral.

A sentence like:

Mi hermano está enfadado porque su cumpleaños lo olvidé.

is possible only in special contexts, when you want strong emphasis on su cumpleaños (his birthday, that is what I forgot). It sounds marked and not like a basic learner sentence.

The simple, natural structure is:

[Main clause] + porque + [reason clause]

Why does olvidé have an accent, and how is it pronounced?

The accent mark (tilde) in olvidé shows where the stress falls and also helps distinguish it from other forms.

olvidé is the yo form (I) of the simple past: I forgot.
It is stressed on the last syllable: ol-vi-DÉ.

Without the accent, olvide would follow normal stress rules (stress on the second‑to‑last syllable) and could be interpreted as another form (e.g. present subjunctive). The accent makes clear both the pronunciation and the tense/person.

Approximate pronunciation (Spain):

[ol-bi-ðe], with a soft d sound in the middle (between English d and th).

Why does cumpleaños end in -s if it’s singular? How do you make it plural?

cumpleaños is a compound noun from the phrase cumplir años (to turn years). It looks plural, but:

It is grammatically singular here: el cumpleaños = the birthday.
It keeps the -s even in singular.

Examples:

Hoy es mi cumpleaños. = Today is my birthday.
Su cumpleaños es mañana. = His birthday is tomorrow.

For the plural, the form usually stays the same and you change the article and the rest of the phrase:

los cumpleaños de mis amigos = my friends’ birthdays
(The word cumpleaños itself doesn’t change form.)

In everyday speech, people also say el cumple as a short, informal version in Spain:

Olvidé su cumple. (very colloquial)

Should it be enfadado conmigo instead of enfadado porque? How do con and porque work here?

They express different ideas and can appear together:

estar enfadado con alguien = to be angry with someone
- Mi hermano está enfadado conmigo. = My brother is angry with me.
estar enfadado porque… = to be angry because… (reason)
- Mi hermano está enfadado porque olvidé su cumpleaños.
  My brother is angry because I forgot his birthday.

You can combine both:

Mi hermano está enfadado conmigo porque olvidé su cumpleaños.
= My brother is angry with me because I forgot his birthday.

In your sentence, the focus is on the reason for his anger, so using porque is exactly right. If you also want to say with whom he is angry, you add conmigo.