A justiça protege todos.

Because justiça is a feminine singular noun, so its definite article is a.

In Portuguese, abstract nouns are often used with the definite article more than in English. So A justiça protege todos is very natural and can mean something like Justice protects everyone or The justice system protects everyone, depending on context.

Without the article, Justiça protege todos, it would sound less neutral and more like a slogan, title, or headline.

It can mean either one, depending on context.

• justiça = justice as a principle or ideal
• a Justiça can also mean the justice system, the courts, or the judiciary

So this sentence could be understood as:

• Justice protects everyone
• The justice system protects everyone

Portuguese often allows this kind of ambiguity when the context makes the intended meaning clear.

Because in Portuguese, every noun has a grammatical gender, and justiça is a feminine noun.

That is why it takes:

• a justiça = the justice
• not o justiça

This is grammatical gender, not biological gender. Many abstract nouns ending in -a are feminine, and justiça follows that pattern.

protege is the 3rd person singular present indicative of the verb proteger (to protect).

So:

• eu protejo = I protect
• você/ele/ela protege = you/he/she protects

In this sentence, the subject is A justiça, which is singular, so protege is the correct form.

Because this sentence is a statement of fact or a general statement, so it uses the present indicative:

• A justiça protege todos. = Justice protects everyone.

proteja is a different form:

• present subjunctive
• or formal imperative

For example:

• Espero que a justiça proteja todos. = I hope that justice protects everyone.
• Que a justiça proteja todos. = May justice protect everyone.

So in your sentence, protege is the normal choice.

Because todos is the masculine plural form, and in Portuguese it is also used for:

• a group of men
• a mixed group
• people in general when gender is unspecified

So todos here means everyone or all people in a general sense.

If you used todas, it would mean all women or all of them if the group were specifically feminine.

Here it means everyone or all people, not everything.

That is a very common distinction in Portuguese:

• todos / todas = everyone, everybody, all people, all of them
• tudo = everything

So:

• A justiça protege todos. = Justice protects everyone.
• A justiça protege tudo. would mean Justice protects everything, which is a different idea.

Because todos is being used on its own as a pronoun meaning everyone or all people.

It works as the direct object of protege:

• A justiça = subject
• protege = verb
• todos = direct object

You could also have forms with an article in other contexts, such as:

• A justiça protege todos os cidadãos. = Justice protects all citizens.
• A justiça protege todos eles. = Justice protects all of them.

But in your sentence, todos alone is completely natural.

Yes. proteger is normally a direct transitive verb.

So you say:

• proteger alguém = to protect someone
• proteger todos = to protect everyone

You do not normally say:

• proteger a todos in neutral standard usage

That said, a todos can appear for emphasis or style in some contexts, but the basic and most straightforward pattern is proteger todos.

In Brazilian Portuguese, protege is pronounced roughly like:

pro-TE-zhi

More closely:

• pro = like pro in English, but shorter
• te = stressed syllable
• ge = the g before e sounds like the s in measure or the zh sound

So the whole sentence sounds approximately like:

a zhus-TEE-sa pro-TE-zhi TO-dus

A more accurate Brazilian pronunciation is:

a ʒusˈtisɐ pɾoˈtɛʒi ˈtodus

Because Portuguese commonly uses the same basic word order as English:

Subject + Verb + Object

So:

• A justiça = subject
• protege = verb
• todos = object

This is the most neutral and natural order.

You could change the structure for emphasis, but then the sentence would sound different, for example:

• Todos são protegidos pela justiça. = Everyone is protected by justice.

That is grammatical too, but it is a passive construction and not the same simple pattern as the original.

It can include women too.

In Portuguese, the masculine plural is used for:

• groups of men
• mixed groups
• generic human groups

So todos often means everyone in a general sense, even if the group includes women.

If you specifically want to emphasize both forms, especially in modern inclusive writing, you may sometimes see:

• todos e todas

But the standard sentence A justiça protege todos already works as a general statement meaning Justice protects everyone.

Yes, it is grammatical and natural.

However, the exact interpretation depends on context:

• as a general principle: Justice protects everyone
• as an institutional meaning: The justice system protects everyone

In everyday life, if someone wants to sound a little more explicit, they might also say:

• A justiça protege todas as pessoas.
• A justiça protege todos os cidadãos.

But your original sentence is perfectly correct and idiomatic.