Se eu encontrar um cupom no aplicativo, a promoção fica ainda melhor.

Because after se when Portuguese talks about a possible future condition, it normally uses the future subjunctive.

So:

• Se eu encontrar... = If I find... / If I happen to find...

Here, encontrar is not functioning like an English infinitive. It is the future subjunctive form of encontrar. For many verbs, that form looks exactly like the infinitive.

Compare with irregular verbs, where you can see the difference more clearly:

• Se eu fizer... = if I do / if I make
• Se eu tiver... = if I have

For this sentence, Se eu encontrar... is the standard and expected form.

Se eu encontro... can sound nonstandard for this kind of future hypothetical idea, or it may suggest a different nuance depending on context or region. If you want to say if I find a coupon in the app, the safest and most natural choice is:

• Se eu encontrar um cupom...

Portuguese often uses the present tense to express a likely result or a general consequence, especially in everyday speech.

So:

• a promoção fica ainda melhor = the promotion gets/is even better

This sounds natural and conversational.

You could also say:

• a promoção ficará ainda melhor

That version is more explicitly future and can sound a bit more formal or deliberate. Both are correct, but fica is very common in spoken Brazilian Portuguese.

Here, ficar means something like to become, to end up, or to get.

So:

• a promoção fica ainda melhor = the promotion becomes even better / the deal gets even better

This is a very common use of ficar in Portuguese. It does not mean to stay here.

Ainda melhor means even better.

• melhor = better
• ainda here adds emphasis, like even or still

So the idea is that the promotion is already good, but finding a coupon would make it even better.

Because no is a contraction of:

• em + o = no

And aplicativo is a masculine singular noun:

• o aplicativo = the app

So:

• no aplicativo = in the app / on the app

If the noun were feminine, you would use na:

• na loja = in the store

Portuguese uses definite articles more often than English does.

So a promoção means:

• the promotion
• the deal
• the offer

In this sentence, it refers to a specific promotion already being discussed or understood from context. In Portuguese, including the article here sounds natural.

You can sometimes omit subject pronouns in Portuguese, but in this sentence eu helps a lot.

That is because encontrar does not clearly show the subject by itself here. The form looks the same as the infinitive, so eu makes the meaning clearer:

• Se eu encontrar um cupom... = clearly if I find a coupon...

In Brazilian Portuguese, subject pronouns are often kept for clarity, so eu sounds very natural.

Because the sentence begins with a conditional clause:

• Se eu encontrar um cupom no aplicativo

Then comes the main clause:

• a promoção fica ainda melhor

In Portuguese, when this kind of if-clause comes first, a comma is normally used.

If you reverse the order, the comma is usually not needed:

• A promoção fica ainda melhor se eu encontrar um cupom no aplicativo.

Yes. A very natural alternative is:

• A promoção fica ainda melhor se eu encontrar um cupom no aplicativo.

The meaning stays the same. The main difference is emphasis:

• Starting with Se eu encontrar... highlights the condition first.
• Starting with A promoção fica... highlights the result first.

Yes. Cupom is masculine:

• um cupom
• o cupom

So in the sentence, um cupom is correct.

If you need the plural, the usual Brazilian Portuguese form is:

• cupons

Not always. In shopping contexts, promoção usually means:

• sale
• special offer
• deal
• promotion in the marketing sense

So in this sentence, a promoção is talking about a commercial offer, not something like a job promotion.