Escribo en el diario.

Because Spanish often leaves out subject pronouns when they are already clear from the verb form.

Escribo already means I write or I am writing, so yo is not needed. Spanish is a pro-drop language, which means subject pronouns are often omitted.

You can still say Yo escribo en el diario if you want to add emphasis, contrast, or clarity.

For example:

• Yo escribo en el diario, pero él no. = I write in the diary, but he doesn’t.

Escribo is the first person singular form of the verb escribir (to write) in the present indicative.

So:

• escribir = to write
• escribo = I write / I am writing

A few other present-tense forms are:

• escribes = you write
• escribe = he/she writes, you write
• escribimos = we write
• escribís = you all write (used in Spain)
• escriben = they write, you all write

It can mean either, depending on context.

In Spanish, the simple present often covers both:

• I write
• I am writing

So Escribo en el diario could mean:

• I write in the diary (habitual action)
• I am writing in the diary (right now)

If you want to make the ongoing action extra clear, Spanish can also use:

• Estoy escribiendo en el diario. = I am writing in the diary.

En means in, on, or sometimes at, depending on context.

Here, escribir en el diario means to write in the diary/journal. The preposition en is used because the writing is being done inside or within the pages of the diary.

So:

• escribir en un cuaderno = to write in a notebook
• escribir en un libro = to write in a book
• escribir en el diario = to write in the diary/journal

In Spanish, articles are used more often than in English.

So where English might sometimes say in diary style or in diary, Spanish normally needs an article or another determiner:

• en el diario = in the diary
• en un diario = in a diary
• en mi diario = in my diary

Just en diario would not sound natural in this meaning.

Both are possible, but they mean slightly different things.

• en el diario = in the diary
• en mi diario = in my diary

Spanish sometimes uses the definite article where English would prefer a possessive, but with something like diary, mi diario is often more natural if you mean your own personal diary.

So:

• Escribo en el diario is grammatical.
• Escribo en mi diario may sound more specific and natural if you are talking about your own diary.

Yes, diario can mean both, depending on context.

Common meanings include:

• diary / journal
• newspaper / daily paper
• as an adjective, daily

In this sentence, because of escribo en el diario, it most naturally means diary/journal if that is the meaning you were given.

But in another context, diario could mean newspaper:

• Leo el diario. = I read the newspaper.

So context matters.

Because that would mean something different.

• Escribo en el diario = I write in the diary
• Escribo el diario = I write the diary / I am writing the newspaper

Without en, el diario becomes the direct object of escribo. That changes the meaning from writing inside the diary to writing/producing the diary itself.

So the preposition is important.

The normal order is Escribo en el diario, but Spanish word order is more flexible than English.

You might also see:

• En el diario escribo.

That version is more marked and may sound literary, contrastive, or emphatic, as if you are stressing where you write.

For everyday neutral speech, Escribo en el diario is the most natural order.

Diario is a masculine singular noun here, so it takes el.

• el diario = the diary / the newspaper

That is why you do not say la diario.

Also:

• nouns ending in -o are often masculine, though not always
• nouns ending in -a are often feminine, though not always

You would change both the noun and the article:

• el diario = the diary
• los diarios = the diaries / the newspapers

And the sentence could become:

• Escribo en los diarios.

Depending on context, that could mean:

• I write in the diaries
• I write in newspapers

In Spain, escribo is pronounced roughly like es-KREE-bo.

A few helpful notes:

• e = like the e in met, but cleaner and tenser
• scri = the stress is on cri
• b in Spanish is softer than English b, though learners can start with a normal b
• the stress falls on cri: es-CRI-bo

Diario is roughly dya-rio or dee-AH-rio, depending on accent and how carefully it is spoken.

Yes.

The difference is:

• en el diario = in the diary / in the journal (a specific one)
• en un diario = in a diary / in a journal (any one, not specific)

So if you are introducing the idea for the first time or speaking more generally, un diario may be a better fit.

Examples:

• Escribo en un diario todos los días. = I write in a diary every day.
• Escribo en el diario que me regalaste. = I write in the diary you gave me.