À tarde, eu prefiro estudar português na biblioteca.

À tarde is a very common time expression in Portuguese meaning in the afternoon. Portuguese often puts time expressions at the beginning of a sentence, just like English can do with phrases like In the afternoon, ....

It could also appear later in the sentence, for example: Eu prefiro estudar português na biblioteca à tarde.

Both are natural.

The à has a grave accent. In this expression, it shows a contraction called crase: historically, it comes from a + a.

In practice, for learners, the important thing is this:

• à tarde
• à noite
• às vezes

These are standard forms you should learn as fixed expressions.

Also, this accent is not the same as the acute accent in words like português.

The comma is used because À tarde has been moved to the front as an introductory time phrase.

Compare:

• À tarde, eu prefiro estudar português na biblioteca.
• Eu prefiro estudar português na biblioteca à tarde.

The first version often uses a comma; the second usually does not need one.

You can often leave it out.

Portuguese is a pro-drop language, which means subject pronouns are frequently omitted when the verb already makes the subject clear.

So both are natural:

• À tarde, eu prefiro estudar português na biblioteca.
• À tarde, prefiro estudar português na biblioteca.

Using eu can add emphasis, contrast, or simply make the sentence sound a little more explicit.

After preferir, Portuguese normally uses an infinitive when the next action is a verb.

So:

• prefiro estudar = I prefer to study
• prefiro ler = I prefer to read
• prefiro dormir = I prefer to sleep

Estudo is a conjugated form, so prefiro estudo would not work here.

Prefiro is the first-person singular present tense of preferir.

So it means:

• I prefer

A quick pattern:

• eu prefiro
• você prefere
• ele/ela prefere
• nós preferimos
• vocês preferem
• eles/elas preferem

In Portuguese, names of languages are usually not capitalized.

So Portuguese writes:

• português
• inglês
• espanhol
• francês

This is different from English, where Portuguese, English, and so on are capitalized.

When talking about studying, speaking, or learning a language, Portuguese usually does not use an article before the language name.

So these are normal:

• estudar português
• falar inglês
• aprender espanhol

Using an article here would usually sound unnatural.

Na is a contraction of em + a.

• em = in, at
• a = the

So:

• na biblioteca = in the library / at the library

Portuguese very often uses the article with places like this, so na biblioteca sounds natural, while em biblioteca would not.

It can mean either, depending on context.

Portuguese em and its contractions like na often cover both ideas that English separates into in and at.

So na biblioteca can refer to location in a general sense:

• physically inside the library
• being at the library as the place where the action happens

In Brazil, de tarde is common in everyday speech, but à tarde is the more standard and neutral form, especially in writing and careful speech.

So:

• à tarde = very standard
• de tarde = common in conversation in many regions

For a learner, à tarde is the safest form to use first.

The acute accent in português shows the stressed syllable and helps with pronunciation.

The stress is on the last syllable:

• por-tu-guês

Without the accent, the spelling would not follow the normal stress rules of Portuguese.

This accent is different from the grave accent in à:

• à = marks crase
• ê / é / ó, etc. = usually mark stress and vowel quality

Yes. Portuguese word order is fairly flexible, especially with time and place expressions.

These are all possible:

• À tarde, eu prefiro estudar português na biblioteca.
• Eu prefiro estudar português na biblioteca à tarde.
• Na biblioteca, eu prefiro estudar português à tarde.

The basic meaning stays similar, but the first element usually gets a bit more emphasis.

Both are possible, but they are not exactly the same.

• estudar português focuses on the activity of studying
• aprender português focuses more on learning/acquiring the language

So in this sentence, estudar português is very natural because it fits the idea of spending time studying, especially na biblioteca.

Yes, but the meaning changes slightly.

• prefiro estudar... = I prefer to study...
• gosto de estudar... = I like studying...

Preferir expresses preference, often comparing one option with others.
Gostar de expresses enjoyment.

So the original sentence is specifically about preference, not just liking something.