Se a reunião terminar cedo, aí eu vou ao mercado.

Se means if. It introduces a condition:

• Se a reunião terminar cedo = If the meeting ends early

So the whole sentence sets up a condition first, and then gives the result:

• If the meeting ends early, then I’m going to the market.

Because after se when you are talking about a future possibility, Portuguese normally uses the future subjunctive.

So in this sentence:

• Se a reunião terminar cedo...

the form terminar is the future subjunctive, not the present indicative.

Compare:

• Se a reunião termina cedo... = sounds like a habitual or repeated situation, something more like If the meeting ends early / when the meeting ends early in a general sense
• Se a reunião terminar cedo... = If the meeting ends early on this future occasion

This is a very important pattern in Portuguese.

That is very common with regular verbs. The future subjunctive often looks identical to the infinitive.

So here:

• terminar could be the infinitive in other contexts
• but after se, in this kind of future condition, it is understood as future subjunctive

You can see the difference more clearly with irregular verbs:

• se eu for = if I go / if I am
• se ele tiver = if he has
• se nós fizermos = if we do / make

Those forms clearly do not look like infinitives, which helps show what is happening grammatically.

In Brazilian Portuguese, the present tense of ir is very commonly used to talk about the near future, especially in everyday speech.

So:

• eu vou ao mercado = I’m going to the market / I’ll go to the market

This is natural and very common.

You could also say:

• eu irei ao mercado

but that sounds more formal, more written, or less conversational in many contexts.

So vou here is completely normal.

Aí here means something like:

• then
• in that case
• so

It connects the result to the condition in a very natural, spoken Brazilian way.

So:

• Se a reunião terminar cedo, aí eu vou ao mercado.

feels a bit like:

• If the meeting ends early, then I’ll go to the market.

It adds a conversational flow. In speech, Brazilians use aí a lot.

Yes. The sentence still works perfectly without it:

• Se a reunião terminar cedo, eu vou ao mercado.

This version is slightly more neutral and direct.

Adding aí makes it sound a bit more conversational and can emphasize the consequence:

• If that happens, then I go to the market.

So both are correct; the difference is mostly in tone and style.

Ao is a contraction of:

• a + o = ao

In standard Portuguese, ir often combines with the preposition a when it means to go to a place.

Since mercado is a masculine singular noun, the article is o, so:

• a + o mercado → ao mercado

This is why you get:

• vou ao mercado = I go / I’m going to the market

A similar example:

• vou à escola = I’m going to school

Here à = a + a.

Because Portuguese uses definite articles much more often than English.

• a reunião = the meeting

In this sentence, the speaker is talking about a specific meeting, probably one both people already know about.

In English, article use is sometimes more flexible, but in Portuguese it is very normal to include the article here.

Cedo is an adverb, meaning early.

It modifies the verb terminar:

• terminar cedo = to end early

It does not change form to match reunião, because adverbs do not agree with nouns.

So even though reunião is feminine, you still say:

• a reunião terminar cedo

not ceda.

Yes. You can put the main clause first:

• Eu vou ao mercado se a reunião terminar cedo.

This means the same thing.

A useful punctuation note:

• If the se clause comes first, a comma is normally used:
  Se a reunião terminar cedo, eu vou ao mercado.
• If the se clause comes second, the comma is usually not needed:
  Eu vou ao mercado se a reunião terminar cedo.

Yes, you can, but there is a small nuance.

• vou ao mercado = the standard, very common way to say I’m going to the market
• vou para o mercado = also possible, often with a bit more emphasis on direction or destination

In everyday Brazilian Portuguese, both can be heard, but vou ao mercado is a very solid choice for learners and matches the sentence you were given.

For this specific meaning, yes—at least in standard grammar, it would not be the best choice.

If you mean a future condition tied to one future event, Portuguese wants the future subjunctive:

• Se a reunião terminar cedo...

Using termina instead would sound more like a general or habitual pattern, depending on context.

So for If the meeting ends early, then I’ll go to the market, the correct form is:

• Se a reunião terminar cedo, aí eu vou ao mercado.