Guarda a chave no bolso para não a perderes.

Because guarda here is the affirmative imperative for tu: a command to one person.

With tu, affirmative commands usually use the same shape as the 3rd person singular present:

• Tu guardas = you keep
• Guarda! = keep! / put away!

So Guarda a chave... means Keep/put the key...

A useful comparison:

• Guarda a chave. = Keep the key.
• Não guardes a chave aí. = Don’t keep the key there.

In the negative, Portuguese uses the subjunctive form, so you get guardes, not guarda.

Because Portuguese often drops subject pronouns when they are already clear from the verb form or the situation.

In a command, the subject is usually obvious, so Guarda a chave... naturally means You, keep the key...

Adding tu is possible, but it usually adds emphasis or contrast:

• Guarda tu a chave. = You keep the key.
• Tu guarda... is not the normal standard pattern.

So leaving out tu is the most natural choice here.

Not usually in this kind of sentence. This is a very common false-friend issue.

Here guardar means something like:

• to keep
• to put away
• to store
• to keep safe

So Guarda a chave no bolso is more like Keep/put the key in your pocket, not Guard the key in your pocket.

Depending on context, guardar can sometimes relate to protection, but in everyday Portuguese it very often means keep or put away.

Because Portuguese uses the definite article very naturally with specific nouns.

So a chave means the key.

In this sentence, the speaker is referring to a specific key, not just any key in general. Portuguese normally includes the article here.

Compare:

• Guarda a chave. = Keep the key.
• Preciso de uma chave. = I need a key.

English sometimes omits articles in places where Portuguese would not, so this is something learners notice a lot.

Because no is a contraction:

• em = in
• o = the
• em + o = no

So:

• no bolso = in the pocket

This contraction is extremely common in Portuguese. Some other examples:

• na casa = in the house
• nos bolsos = in the pockets
• nas mãos = in the hands

Because Portuguese often uses the definite article where English prefers a possessive.

So no bolso can naturally mean in the pocket, but in context it is understood as in your pocket.

This is very common with:

• body parts
• clothing
• things closely associated with someone

For example:

• Doe-me a cabeça. = My head hurts.
• Meteu as mãos nos bolsos. = He put his hands in his pockets.

So in this sentence, no bolso is perfectly natural even though English would usually say in your pocket.

Para introduces purpose. In this sentence, para não means:

• so as not to
• in order not to
• so that you don’t

So the structure is:

• Guarda a chave no bolso = Keep the key in your pocket
• para não a perderes = so that you don’t lose it

The idea is: do the first action with the purpose of avoiding the second.

It is a direct object pronoun meaning it, referring back to a chave.

Because chave is feminine singular, the pronoun is a:

• a chave → a = it

So:

• para não a perderes = so as not to lose it

This is the same kind of pronoun as in:

• Vi a Maria. → Vi-a. = I saw Maria. → I saw her.
• Tenho a chave. → Tenho-a. = I have the key. → I have it.

Because this is the personal infinitive, one of the special features of Portuguese.

The infinitive can sometimes show who the subject is. Here:

• perder = to lose
• perderes = for you to lose

Since the understood subject is tu, Portuguese uses perderes.

So para não a perderes literally corresponds to something like:

• so as for you not to lose it

In natural English, of course, you would just say so that you don’t lose it.

This personal infinitive is very common after words like para.

Because não attracts the object pronoun to a position before the verb. This is called proclisis.

So:

• a = it
• não a perderes = not lose it

The negative word não is one of the most important triggers for this pattern.

That is why the sentence has:

• para não a perderes

rather than a post-verbal pronoun arrangement.

So the order is natural European Portuguese:

• não + pronoun + verb

Yes. That is also correct and natural.

It would mean the same thing:

• para não perderes a chave = so that you don’t lose the key
• para não a perderes = so that you don’t lose it

The version with a simply avoids repeating a chave.

So the sentence in your example sounds a bit more elegant and less repetitive, but both versions are fine.

Yes. A possible alternative is:

• Guarda a chave no bolso para que não a percas.

This uses para que + subjunctive.

Both are correct, but para + infinitive is usually more direct and very natural here, especially when the subject is clear. That is why para não a perderes is such a good, idiomatic choice in Portuguese.