Trabajo en la oficina hasta la medianoche.

In Spanish, subject pronouns are often omitted because the verb ending already tells you who the subject is.

• Trabajo = I work
• The -o ending shows it is yo

So Trabajo en la oficina... is completely natural. You could say Yo trabajo en la oficina... if you want extra emphasis, contrast, or clarity, but in normal conversation the yo is usually left out.

Trabajo is the first person singular present tense of trabajar (to work).

Conjugation of trabajar in the present:

• yo trabajo = I work
• tú trabajas = you work
• él/ella trabaja = he/she works
• nosotros trabajamos = we work
• ustedes trabajan = you all work
• ellos trabajan = they work

In this sentence, trabajo can mean either:

• I work in the office until midnight (habitual)
• I am working in the office until midnight (depending on context)

Spanish simple present often covers both ideas.

Because en means in / at, which fits the idea of location.

• Trabajo en la oficina = I work in / at the office
• Voy a la oficina = I go to the office

So:

• en = being located somewhere
• a = movement toward a place

That is why en la oficina is correct here.

Yes, oficina usually means office, especially a workplace or office space.

Examples:

• Trabajo en una oficina. = I work in an office.
• La oficina está cerrada. = The office is closed.

Just be aware that Spanish sometimes uses other words depending on context, but oficina is the normal word here.

Usually, yes. Oficina is a feminine noun, and many nouns ending in -a are feminine.

So:

• la oficina
• una oficina

That said, noun gender is not always perfectly predictable in Spanish, so it is best to learn nouns together with their article:

• la oficina
• el trabajo
• la medianoche

Hasta means until in this sentence.

• Trabajo en la oficina hasta la medianoche. = I work in the office until midnight.

It marks the endpoint in time.

More examples:

• Estudio hasta las diez. = I study until ten.
• Esperamos hasta mañana. = We wait until tomorrow.

In some contexts, hasta can also mean up to or even as far as, but here it clearly means until.

In Spanish, times of day and certain time expressions often use the definite article.

So la medianoche = midnight

You will also see:

• la mañana = the morning
• la tarde = the afternoon/evening
• la noche = the night

With clock expressions, Spanish also often uses articles:

• Es la una.
• Son las dos.

So hasta la medianoche sounds natural and standard.

Yes. Medianoche is written as one word.

It comes from:

• media = half
• noche = night

But together, medianoche means midnight.

A related word is:

• mediodía = noon

Both are commonly written as single words.

Yes. Spanish present tense is often broader than English present tense.

Trabajo en la oficina hasta la medianoche can mean:

• I work in the office until midnight (habit/routine)
• I’m working in the office until midnight (current situation or plan), depending on context

If someone wants to emphasize the ongoing action right now, they might also say:

• Estoy trabajando en la oficina hasta la medianoche.

But the simple present is very common and natural.

A broad Latin American-style pronunciation would sound roughly like:

tra-BA-ho en la o-fi-SI-na AS-ta la me-dya-NO-che

A few helpful notes:

• j in trabajo is like a strong breathy h
• h in hasta is silent
• ll is not in this sentence, but many learners confuse it with y
• c before i in oficina sounds like s in Latin America
• ch in medianoche is like English ch

A rough English-friendly approximation: trah-BA-ho en lah oh-fee-SEE-nah AHS-tah lah meh-dyah-NOH-che

Yes, it sounds natural.

Trabajo en la oficina hasta la medianoche is a correct and normal sentence in Latin American Spanish.

Depending on context, native speakers might also say:

• Trabajo en la oficina hasta medianoche.
• Estoy en la oficina trabajando hasta la medianoche.
• Hoy trabajo hasta la medianoche.

But your original sentence is fully natural and grammatically correct.

They mean different things.

• hasta la medianoche = until midnight
• por la medianoche = around midnight / at about midnight

So:

• Trabajo en la oficina hasta la medianoche. = I work in the office until midnight.
• Llego por la medianoche. = I arrive around midnight.

This is a very important distinction because hasta gives an endpoint, while por gives an approximate time.

Normally, no. You would usually say en la oficina or en una oficina.

• en la oficina = in the office / at the office
• en una oficina = in an office

Leaving out the article in this sentence sounds incomplete or unnatural in standard Spanish. Spanish uses articles more often than English does in many cases, so it is best to include la here.