La sala del documental está casi llena, así que llegamos justo a tiempo.

Sala usually means room, hall, or screening room, depending on context. Here it most likely refers to the place where the documentary is being shown.

So la sala del documental is literally something like the room/hall of the documentary, but in more natural English you might think of it as the screening room for the documentary or the auditorium where the documentary is showing.

Spanish often uses a de + noun structure where English might prefer a different phrasing.

Because del is the mandatory contraction of de + el.

• de + el = del
• a + el = al

So:

• la sala del documental = the room of/from the documentary

You normally do not say de el documental unless él is the pronoun he/him, which is a different word.

Spanish uses estar for states or conditions, especially ones seen as temporary or changeable.

Here, casi llena describes the current state of the room: it is almost full right now. That is why está is used:

• La sala está casi llena.

If you used es, it would sound wrong in this context, because being full is not treated as a permanent characteristic of the room.

Because llena agrees with la sala, which is a feminine singular noun.

Agreement in Spanish:

• la sala → feminine singular
• adjective must match → llena

Compare:

• El cine está lleno.
• La sala está llena.
• Las salas están llenas.

So the ending changes to match the noun.

Casi means almost or nearly.

It comes before the word it modifies, just like in English:

• casi llena = almost full
• casi listo = almost ready
• casi nunca = almost never

So está casi llena means the room has not become completely full yet, but it is very close.

Así que means so, therefore, or which means that.

It introduces a result or consequence:

• La sala está casi llena, así que... = The room is almost full, so...

It is a very common way to link two ideas when the second follows logically from the first.

It is different from porque, which means because and gives a reason rather than a result.

• Llegamos tarde porque había tráfico. = We arrived late because there was traffic.
• Había tráfico, así que llegamos tarde. = There was traffic, so we arrived late.

Because Spanish often drops subject pronouns when the verb already makes the subject clear.

Llegamos already tells you the subject is we.

So both are possible:

• Llegamos justo a tiempo.
• Nosotros llegamos justo a tiempo.

But the version without nosotros is more natural unless you want emphasis or contrast.

For example:

• Ellos llegaron tarde, pero nosotros llegamos justo a tiempo.

There, nosotros is useful because it contrasts we with they.

Grammatically, llegamos can be either:

• present: we arrive
• preterite: we arrived

This happens with many -ar verbs in the nosotros form.

Here, context tells you it is most naturally preterite: we arrived / we got there just in time. The arrival is understood as a completed action.

So although the form is ambiguous by itself, the sentence makes the past meaning the most likely one.

Justo a tiempo is the standard expression for just in time or right on time.

• Llegamos justo a tiempo. = We arrived just in time.

Here justo does not mean fair. It means something like exactly or right.

You can also hear:

• justo antes = just before
• justo después = just after
• justo ahora = right now

Because así que links two full clauses, and Spanish normally separates them with a comma in this kind of sentence:

• La sala del documental está casi llena, así que llegamos justo a tiempo.

The comma helps mark the pause between:

1. the situation: the room is almost full
2. the result: so we arrived just in time

This is very similar to English punctuation with so in many sentences.

In most of Spain today, ll is usually pronounced the same as y. So llena sounds roughly like yena to an English speaker.

However, pronunciation varies by region. In some areas, speakers preserve a distinction between ll and y, but that is much less common.

For most learners of Spanish from Spain, pronouncing llena like yena is perfectly normal.