Na universidade, os alunos usam crachá para entrar no prédio e no setor da biblioteca.

Na universidade means in/at the university. The word na is a contraction of em + a.

• em = in, on, at
• a = the, feminine singular

Since universidade is a feminine noun, em a universidade becomes na universidade.

Starting the sentence this way sets the scene, like At the university, ... in English.

Because universidade is feminine.

In Portuguese, articles must match the gender of the noun:

• masculine singular: o
• feminine singular: a

So:

• em + o = no
• em + a = na

Since it is a universidade, the correct form is na universidade.

Portuguese often uses definite articles more than English does. So os alunos literally means the students, but in many contexts it can sound natural even when English might simply say students.

Here, os alunos refers to the students in that university context, so it sounds very normal.

Because the subject is os alunos, which is third person plural.

The verb usar must agree with the subject:

• eu uso
• você / ele / ela usa
• nós usamos
• vocês / eles / elas usam

Since os alunos = they / the students, the correct form is usam.

Crachá is a masculine noun and usually means ID badge, name badge, or access badge, depending on context.

In this sentence, it most likely refers to an identification/access badge used to enter places on campus.

It is singular here, but Portuguese often uses a singular noun after a verb like this in a general sense:

• usar crachá = to wear/use a badge

That sounds natural, just like saying wear a uniform or use protection in some contexts.

In Portuguese, nouns often appear without an article when speaking about something in a general or functional way.

So usam crachá means something like:

• they use/wear a badge
• they wear badges as part of the normal rule or practice

If you said usam o crachá, it would sound more like a specific badge already known in the conversation.
If you said usam um crachá, it would emphasize a badge as one item.

So usam crachá is very natural for a general rule or routine.

Para here expresses purpose: in order to or to.

So:

• usam crachá para entrar = they use a badge to enter

After para, Portuguese commonly uses the infinitive when the subject stays the same:

• para estudar
• para trabalhar
• para entrar

This is very similar to English to enter.

Because Portuguese normally contracts em + o into no.

So:

• em o prédio → no prédio

This contraction is required in standard Portuguese.

The same pattern appears in many places:

• em + a = na
• em + os = nos
• em + as = nas

So entrar no prédio literally has in the building, but idiomatically it means enter the building.

Because the verb entrar applies to both places:

• entrar no prédio
• entrar no setor da biblioteca

The sentence links the two with e:

• ... para entrar no prédio e no setor da biblioteca

The second no is repeated because each destination needs its own preposition + article combination.

This is very common in Portuguese and sounds more natural than leaving it out.

Setor usually means sector, section, area, or sometimes department, depending on context.

Da biblioteca is de + a biblioteca, meaning of the library.

So o setor da biblioteca is something like:

• the library section
• the library area
• the library department

The exact English wording depends on the institution. In a university context, it probably means a specific area or department connected to the library.

Because da is the contraction of de + a.

• de = of, from
• a = the, feminine singular

Since biblioteca is feminine, de a biblioteca becomes da biblioteca.

This is another very common contraction in Portuguese:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

Yes. A very natural alternative would be:

• Os alunos usam crachá para entrar no prédio e no setor da biblioteca, na universidade.

But the original version, with Na universidade at the beginning, is especially useful for setting the context first.

Portuguese is often flexible with word order, especially with time/place expressions like:

• na universidade
• hoje
• de manhã
• no Brasil

Putting them at the beginning often gives the sentence a nice contextual frame.

Crachá is pronounced roughly like crah-SHAH in Brazilian Portuguese.

The written accent on á shows that the stress falls on the last syllable:

• cra-chá

Without the accent, Portuguese spelling rules would suggest a different stress pattern, so the accent is important.

Yes, in spoken Brazilian Portuguese, pra is extremely common as a contraction of para.

So you will often hear:

• usam crachá pra entrar

That sounds natural in speech and informal writing.
But para entrar is more neutral and standard, especially in textbooks, formal writing, or careful speech.

It can correspond to either in the university or at the university in English, depending on context.

Portuguese em covers several location meanings that English splits into in, on, and at. So na universidade is best understood as a general location phrase meaning at the university here.

English and Portuguese do not always match word-for-word with prepositions, so it is better to learn na universidade as the natural Portuguese expression.