Llegamos a medianoche.

Llegamos comes from the verb llegar, meaning to arrive. It is the first person plural form, so the subject is we.

That means the sentence has an understood subject:

• (Nosotros) llegamos a medianoche.

Spanish often leaves out the subject pronoun because the verb ending already shows who is doing the action.

Yes. This is a very common point of confusion.

For many -ar verbs, the we form of the present tense and the preterite looks exactly the same:

• llegamos = we arrive
• llegamos = we arrived

So without context, Llegamos a medianoche could mean either:

• We arrive at midnight
• We arrived at midnight

Usually, context tells you which one is meant:

• If you are talking about a schedule or a habitual action, it is probably present.
• If you are telling a story about something completed, it is probably preterite.

Because Spanish often drops subject pronouns.

The ending -amos already tells you the subject is we, so nosotros is not necessary.

Spanish speakers usually include nosotros only when they want:

• emphasis
• contrast
• clarity

For example:

• Llegamos a medianoche. = neutral
• Nosotros llegamos a medianoche. = emphasizes we

Because Spanish normally uses a with specific times or moments when something happens.

So you get:

• a las ocho
• a mediodía
• a medianoche

In this sentence, a medianoche means at midnight.

A native English speaker may want to use en, but that would not be natural here. For time points like this, a is the normal choice.

A medianoche is a very common fixed expression meaning at midnight.

Even though the noun is normally la medianoche, the article is often left out in this time expression. So:

• la medianoche = midnight
• a medianoche = at midnight

You may sometimes see a la medianoche, and it can be understood, but a medianoche is the more standard, idiomatic way to say it in a simple sentence like this.

Yes. Medianoche is written as one word.

It is a noun, and it is normally feminine:

• la medianoche

It is built from:

• media = half / middle
• noche = night

So the idea is literally the middle of the night, but in normal use it means midnight.

That comes from the meaning of medianoche and the context.

The preposition a can be used in different ways in Spanish, but here medianoche is clearly a time word, so a medianoche is understood as at midnight, not to midnight.

Spanish does this often:

• a las tres
• a la una
• a medianoche

So in this sentence, a is introducing the time of arrival.

A simple approximate pronunciation for an English speaker is:

yeh-GAH-mos a meh-dya-NO-che

The main stress falls on:

• ga in llegamos
• no in medianoche

A more careful pronunciation is roughly:

• llegamos → yeh-GAH-mos
• medianoche → meh-dya-NO-che

Depending on the region in Spain, the ll sound in llegamos may sound like an English y to your ear.

Yes. That is also correct and natural.

Compare:

• Llegamos a medianoche.
• Llegamos a las doce de la noche.

Both can mean the same thing.
A medianoche is shorter and more idiomatic.
A las doce de la noche sounds a little more like you are stating the clock time explicitly.

Very similarly, yes.

Compare:

• Llegamos a medianoche. = We arrived / arrive at midnight
• Llegamos a mediodía. = We arrived / arrive at noon

Both medianoche and mediodía often appear without an article after a in fixed time expressions:

• a medianoche
• a mediodía