Guarde a carta de jogo, vai que você precisa.

Because this is a command directed at você.

In Brazilian Portuguese, when you give an affirmative command to você, you normally use the present subjunctive form:

• guardar → guarde

So Guarde a carta de jogo means Keep the game card or Put the card away.

If you were talking to tu, the command would usually be:

• Guarda a carta...

So the form guarde matches the later você in the sentence.

Portuguese often leaves out subject pronouns when they are understood from the verb or the context.

So:

• Guarde a carta... = (Você) guarde a carta...

The subject is still you, but it does not have to be said.

In the second part, você is included because the speaker is stating a whole clause after vai que, and saying the pronoun makes it sound natural and clear:

• vai que você precisa

So this is very normal: omitted in one clause, explicit in the next.

Vai que is a very common colloquial Brazilian expression. It does not mean its literal word-for-word meaning.

Here it means something like:

• what if
• in case
• you never know
• maybe you’ll end up needing it

So:

• Guarde a carta de jogo, vai que você precisa.

has the sense of:

• Keep the game card — you might need it.
• Keep the card, just in case you need it.

It is often used to justify a precaution.

That is because vai que is a colloquial expression that very often takes the present indicative in everyday Brazilian Portuguese:

• vai que chove
• vai que dá certo
• vai que você precisa

Even though the meaning is about a possible future situation, speakers commonly use the present form.

A more formal or more textbook-style version might be:

• caso você precise
• se você precisar

So:

• vai que você precisa = colloquial, natural in speech
• caso você precise = more formal / more standard written style

Yes, if you want to be fully explicit, you could say:

• vai que você precisa dela

where dela means of it / of her, referring to a carta.

But in Portuguese, especially in everyday speech, speakers often leave out the object when it is obvious from context.

So vai que você precisa is understood as:

• vai que você precisa da carta
• vai que você precisa dela

This kind of omission is very common in natural conversation.

It is understandable, but the most natural phrasing depends on the context.

Possible options:

• carta de baralho = a playing card
• carta do jogo = a card from a specific game
• carta de jogo = understandable, but less fixed/common than the two above

So if the meaning is a regular playing card, many Brazilians would more naturally say:

• Guarde a carta de baralho...

If it is a card belonging to a particular game, then:

• Guarde a carta do jogo...

The sentence you gave still works, but a learner might notice that there are more idiomatic alternatives.

Because Portuguese usually uses articles more often than English, especially when referring to a specific thing already known in the situation.

So a carta de jogo means:

• the game card
• the card from the game

The article a shows that the speaker has a particular card in mind, not just any card in general.

Without the article, the sentence would usually sound less natural in this context.

It is mostly informal to neutral, especially because of vai que.

• Guarde... by itself can sound neutral or polite.
• vai que... is clearly conversational and common in spoken Brazilian Portuguese.

So the whole sentence sounds like something someone might casually say in real life.

A more formal version could be:

• Guarde a carta do jogo, caso precise dela.

That sounds more polished and less conversational.

The comma separates the main command from the reason or justification that follows.

• Guarde a carta de jogo = the instruction
• vai que você precisa = the speaker’s justification: you might need it

So the comma helps show the pause:

• Keep the card, just in case you need it.

In speech, there is usually a small pause there too.