Nem todos gostam de estudar na biblioteca.

Nem todos means not everyone or not all people.

In this sentence, nem is used to create a partial negative idea: some people like studying in the library, but others do not.

So:

• Nem todos gostam... = Not everyone likes...
• It does not mean that nobody likes it.

Using não directly before todos would sound unnatural here. Nem todos is the normal expression in Portuguese for not all / not everyone.

Because the subject is todos, which is plural.

Even though the sentence is negative in meaning, the grammatical subject is still todos = all people / everyone, and that idea is plural in form.

So:

• todo → singular
• todos → plural

That is why the verb is:

• gostam = they like

If the subject were singular, you would use gosta:

• Nem todo aluno gosta... = Not every student likes...

Because gostar normally requires the preposition de in Portuguese.

So the pattern is:

• gostar de + noun
• gostar de + infinitive

Examples:

• Eu gosto de música.
• Ela gosta de ler.
• Eles gostam de estudar.

For an English speaker, this is important because English says like something / like doing something with no preposition, but Portuguese usually needs de after gostar.

After a preposition such as de, Portuguese normally uses the infinitive.

So:

• gostar de estudar = to like studying / to like to study

Portuguese does not use the gerund here the way English often does. English says like studying, but Portuguese prefers:

• gostar de estudar

This is very common:

• antes de sair = before leaving
• depois de comer = after eating
• sem falar = without speaking

So de estudar is the normal structure.

Na is a contraction of:

• em + a = na

Here, biblioteca is a feminine noun, so a biblioteca means the library.

Therefore:

• na biblioteca = in the library / at the library

This kind of contraction is very common in Portuguese:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

Examples:

• no carro = in the car
• na escola = at school / in the school

Because todos can work by itself as a pronoun, meaning everyone or all people.

So in this sentence:

• Nem todos gostam... = Not everyone likes...

The noun is not stated, but it is understood from context as all people.

If you want to name the group, you can add a noun:

• Nem todos os alunos gostam de estudar na biblioteca.
• Nem todos os estudantes gostam de estudar na biblioteca.

Without a noun, the sentence is more general.

Yes. A few natural alternatives are:

• Nem todo mundo gosta de estudar na biblioteca.
• Não é todo mundo que gosta de estudar na biblioteca.

These also mean not everyone likes studying in the library.

Some notes:

• Nem todos gostam... sounds a bit more neutral or written.
• Nem todo mundo gosta... is very common in everyday speech.
• Não é todo mundo que... is also very common and conversational.

This is an important difference:

• Nem todos gostam de estudar na biblioteca. = Not everyone likes studying in the library.
• Ninguém gosta de estudar na biblioteca. = Nobody likes studying in the library.

So:

• nem todos = partial negation
• ninguém = total negation

With nem todos, some people may still like it. With ninguém, zero people like it.

Either can be correct, depending on context.

• in the library focuses more on being inside the place
• at the library can sound a little more general in English

Portuguese na biblioteca can cover both ideas in many situations. The Portuguese sentence itself is completely natural.

Because after gostar de, Portuguese uses the infinitive, not the gerund.

So:

• gostar de estudar = correct
• gostar de estudando = incorrect

English often uses -ing after like, but Portuguese does not do that here.

Compare:

• I like studying.
• Eu gosto de estudar.

That is one of the most common structure differences between English and Portuguese.

Yes. In Portuguese, the masculine plural form todos is normally used for:

• a group of men
• a mixed group of men and women
• people in general

If the group is all female, you can use todas:

• Nem todas gostam de estudar na biblioteca.

So the sentence with todos can be general and does not necessarily mean only males.

A natural spoken rhythm would usually stress the meaningful words:

Nem TODOS gostam de estudar na biblioteCA.

Often todos gets emphasis if you want to highlight the idea of not everyone.

If you want to contrast opinions, you might stress it even more:

• Nem TODOS gostam de estudar na biblioteca.

That emphasizes that some do, some do not.

Yes, but the original order is the most normal and natural.

Standard order:

• Nem todos gostam de estudar na biblioteca.

You might also hear other structures for emphasis, but they are less neutral, for example:

• De estudar na biblioteca, nem todos gostam.

This sounds more marked or literary. For most situations, the original sentence is best.