En mi diario escribo un breve poema cada noche sobre mi estado de ánimo y mi humor.

Both word orders are correct:

• En mi diario escribo un breve poema…
• Escribo en mi diario un breve poema…

Spanish word order is more flexible than English. Putting En mi diario first just emphasizes where this action happens, like:

• In my journal, I write a short poem…

So it’s a stylistic choice for emphasis, not a grammar rule.

• mi diario = my journal (a personal possession)
• el diario = the newspaper or the diary in a more general sense

Using mi makes it clearly personal: your own journal. Spanish normally does not use an article when there is a possessive:

• ✅ mi diario
• ❌ el mi diario (incorrect)

diario can be:

1. Noun

   • el diario = the diary / the journal
   • el diario (also) = the newspaper

2. Adjective

   • rutina diaria = daily routine
   • tarea diaria = daily homework

In En mi diario escribo…, diario is a noun: my journal. Context tells you it’s not “newspaper” here, because people don’t usually write poems in a newspaper they own.

In Spanish, the verb ending already shows the subject:

• escribo = I write
• escribes = you (tú) write
• escribe = he/she/you (usted) write(s)

So you normally say:

• Escribo un breve poema… (I write a short poem…)

You only add yo for emphasis or contrast:

• Yo escribo un poema, pero tú lees novelas.
  I write a poem, but you read novels.

poema is one of a group of masculine nouns that end in -a, many coming from Greek:

• el poema – un poema (the poem – a poem)
• el problema – un problema
• el sistema – un sistema
• el tema – un tema

So you must use masculine articles and adjectives with poema:

• un poema breve
• este poema
• el poema famoso

Both are correct:

• un breve poema
• un poema breve

General idea:

• Adjective after the noun (more neutral, objective):

  • un poema breve = a poem that is short (just describing it)

• Adjective before the noun can feel:

  • more literary or stylistic
  • a bit more subjective or expressive

So un breve poema sounds slightly more elegant or literary, but in everyday speech many people would naturally say un poema breve or un poema corto.

Yes, you can say:

• un poema breve
• un poema corto

Both are correct and both mean “a short poem.” Nuance:

• breve often sounds:

  • slightly more formal/literary
  • focused on duration or conciseness

• corto is very common and neutral:

  • un texto corto, un video corto, un viaje corto

un breve poema just gives a more refined or literary tone than un poema corto, but either is fine.

You could say either:

• cada noche = each night
• todas las noches = every night / all nights

Meaning is almost the same in this context. Nuance:

• cada noche highlights the repetition one by one: each individual night
• todas las noches sounds like a general habit: every night, as a whole pattern

In this sentence, both are natural:

• …escribo un breve poema cada noche…
• …escribo un breve poema todas las noches…

sobre here means about:

• un poema sobre mi estado de ánimo = a poem about my mood

You have a few options:

• sobre – very common and natural:
  
  • un libro sobre historia – a book about history
• de – can also mean “about,” more general:
  
  • un poema de amor – a poem about love
• acerca de – slightly more formal:
  
  • un artículo acerca del clima – an article about the climate

So you could say:

• …un breve poema acerca de mi estado de ánimo… (more formal)
• …un breve poema de mi estado de ánimo… (possible but less natural here; sounds more like the poem belongs to my mood than that it is about it)

sobre is the most straightforward and natural choice.

Literally:

• estado = state / condition
• ánimo = spirit, mood, energy

So estado de ánimo = state of mind / emotional state / mood.

Differences:

• ánimo alone can mean:

  • ¡Ánimo! – Cheer up! / Go for it!
  • No tengo ánimo para salir. – I don’t feel like going out.

• estado de ánimo is more clearly the overall emotional state:

  • Mi estado de ánimo ha mejorado. – My mood has improved.

In your sentence, estado de ánimo sounds a bit more formal or precise than just ánimo.

They overlap, but there are nuances:

• humor in Spanish often means mood:
  • Estoy de buen humor. – I’m in a good mood.
  • Está de mal humor. – He/She is in a bad mood.

It can also mean “sense of humor”:

• Tiene buen humor. – He/She has a good sense of humor.

In mi estado de ánimo y mi humor, the writer is probably:

• emphasizing both their general emotional state (estado de ánimo)
• and their mood / temper / sense of humor (humor)

It’s a bit stylistic and repetitive on purpose, like saying "my mood and my overall emotional state" in English. In real life, many people would just pick one:

• …sobre mi estado de ánimo.
• …sobre mi humor.

You can say:

• mi estado de ánimo y humor

But repeating mi is more natural and clearer:

• mi estado de ánimo y mi humor

General pattern:

• It’s common to drop the second mi when it’s very obvious:

  • Mi padre y madre = My father and (my) mother

• But with slightly more complex phrases, repeating mi helps:

  • mi estado de ánimo y mi humor
  • mi salud física y mi salud mental

So the original version with both mi’s sounds nicer and avoids ambiguity.

Spanish uses e instead of y only before words that start with the sound /i/:

• padre e hijo (not y hijo)
• agua e hielo

With humor, the initial sound is /u/, like u-mor, because the h is silent:

• y humor is correct
• e humor would be wrong

So mi estado de ánimo y mi humor is the right form.