A última vez que eu fui ao teatro, a fila já estava longa.

Because Portuguese normally uses the definite article a with this expression.

• a última vez = the last time
• que = that / when in this kind of structure

So A última vez que eu fui ao teatro... is the natural way to say The last time I went to the theater...

Leaving out a would sound incomplete in standard Portuguese.

Here que links a última vez to the clause that describes that time:

• a última vez = the last time
• que eu fui ao teatro = that I went to the theater

So que works like that or sometimes when in English. Portuguese uses que very often in this structure.

Fui is the preterite form of ir for eu:

• eu fui = I went

The preterite is used because this is a completed past event: one specific occasion when the speaker went to the theater.

So:

• eu fui ao teatro = I went to the theater

It can be from either verb, depending on context:

• fui from ir = I went
• fui from ser = I was

In this sentence, it is clearly from ir, because of ao teatro:

• fui ao teatro = I went to the theater

Using ser there would not make sense.

Because a + o contracts to ao in Portuguese.

• ir a = to go to
• o teatro = the theater
• ir ao teatro = to go to the theater

This kind of contraction is very common:

• a + o = ao
• a + a = à
• de + o = do
• em + o = no

So fui ao teatro is the standard form.

In everyday Brazilian speech, some people do say ir no teatro, but standard Portuguese strongly prefers ir ao teatro when you mean go to the theater.

A useful distinction is:

• ir ao teatro = go to the theater
• estar no teatro = be in the theater

So for learners, ao teatro is the safest and most standard choice.

Because estava describes the state of the line at that moment, not a completed event.

• estava is the imperfect of estar
• it gives background information: what the situation was like

So the sentence has this pattern:

• eu fui ao teatro = completed action
• a fila já estava longa = ongoing state at that time

If you said foi longa, it would sound more like you are treating being long as a finished event, which is not the natural idea here.

Because estar is used for a condition or state, especially one that can change.

Here, the line being long is a temporary situation:

• earlier, the line may have been short
• then it became long
• later, it may have changed again

So estava longa is the natural choice.

Using ser would sound wrong here, because ser is not normally used for this kind of temporary condition.

Já means already or by then.

So a fila já estava longa means the line was not just long at that moment—it had already become long before or by the time the speaker got there.

It adds the idea of timing:

• when I arrived, the line was already long

Because longa agrees with fila, which is a feminine noun.

• a fila = feminine singular
• adjective must match: longa

So:

• a fila longa
• not a fila longo

This is standard adjective agreement in Portuguese.

Yes. Fila is the normal word for a line / queue, especially of people waiting.

Examples:

• fila do cinema = movie line
• fila do banco = bank line
• fila do teatro = theater line

English speakers sometimes think of linha, but linha usually means a line in other senses, such as a drawn line, a phone line, a bus line, or a line of text. For people waiting, fila is the right word.

Yes, longa works well here.

With fila, both longa and comprida can be understood, but fila longa is very natural and common. It means the queue had many people or extended a lot.

So a fila já estava longa is a normal way to describe a long line.

Yes, it could be omitted.

Portuguese often drops subject pronouns when the verb form already makes the subject clear:

• A última vez que fui ao teatro...
• A última vez que eu fui ao teatro...

Both are correct.

Including eu can add a bit of emphasis, clarity, or a more conversational feel. In this sentence, either version sounds natural.

Because the sentence starts with a time clause:

• A última vez que eu fui ao teatro = time/background information
• a fila já estava longa = main statement

The comma separates that introductory clause from the main clause. This is very common and helps readability.

Yes, that is possible:

• Quando eu fui ao teatro pela última vez, a fila já estava longa.

But it is a little less direct and less idiomatic than A última vez que eu fui ao teatro... for this exact meaning.

The original sentence is the most natural way to say The last time I went to the theater...

Here it means most recent.

• a última vez que eu fui ao teatro = the most recent time I went to the theater

Portuguese último / última can sometimes mean final, but in this sentence the context clearly shows most recent.