Hoy he estudiado bastante en la biblioteca.

In Spain Spanish, he estudiado is very common for something that happened today, because today is seen as a time period that is still connected to the present.

So:

• Hoy he estudiado... = I have studied today...
• Ayer estudié... = I studied yesterday...

This is the present perfect:
he + past participle

In much of Latin America, people would often say Hoy estudié... instead, but in Spain, Hoy he estudiado... is especially natural.

No. Here, he is not the English-style pronoun he.

It is the first-person singular form of the verb haber, used as an auxiliary verb to make the present perfect.

So:

• he estudiado = I have studied
• has estudiado = you have studied
• ha estudiado = he/she has studied

This he is pronounced like eh, and the h is silent in Spanish.

Spanish often omits subject pronouns because the verb form already shows who the subject is.

• he estudiado already tells you the subject is I
• so yo is not necessary

You could say:

• Hoy he estudiado bastante en la biblioteca.
• Hoy yo he estudiado bastante en la biblioteca.

But the second version usually sounds more emphatic, as if you are stressing I specifically.

Here, bastante means quite a lot, a lot, or fairly much, depending on context.

In this sentence:

• He estudiado bastante = I’ve studied quite a lot

It is acting as an adverb, modifying estudiado.

A few useful related examples:

• He comido bastante. = I’ve eaten quite a lot.
• Es bastante difícil. = It’s quite difficult.

So bastante can mean slightly different things depending on what it modifies, but the general idea is enough / quite a bit / rather.

Because en shows location: where the studying happened.

• en la biblioteca = in/at the library

If you said a la biblioteca, that would usually suggest movement toward the library, not being there.

Compare:

• He estudiado en la biblioteca. = I studied at the library.
• He ido a la biblioteca. = I went to the library.

So:

• en = location
• a = direction/destination

Putting hoy first is very natural because it sets the time frame immediately.

• Hoy he estudiado bastante en la biblioteca.

This feels a bit like: As for today, I’ve studied quite a lot at the library.

Spanish word order is flexible, so you could also say:

• He estudiado bastante hoy en la biblioteca.
• He estudiado hoy bastante en la biblioteca.
• En la biblioteca he estudiado bastante hoy.

But the original sentence is a very normal, clear order.

Yes, grammatically you could, but in Spain, Hoy he estudiado... is usually the more natural choice.

Why?

Because hoy belongs to a time period that is still ongoing. In Spain, that often triggers the present perfect.

So:

• Hoy he estudiado... = very natural in Spain
• Hoy estudié... = possible, but less standard for many speakers in Spain

In many Latin American varieties, Hoy estudié... is much more common.

Because after haber in the present perfect, Spanish uses a past participle, not a gerund.

So:

• he estudiado = correct
• he estudiando = incorrect

Forms:

• infinitive: estudiar
• past participle: estudiado
• gerund: estudiando

Use them like this:

• He estudiado mucho. = I have studied a lot.
• Estoy estudiando. = I am studying.

So:

• haber + participle for have studied
• estar + gerund for am studying

For most -ar verbs, the past participle ends in -ado.

So:

• estudiar → estudiado
• hablar → hablado
• trabajar → trabajado

For most -er and -ir verbs, it ends in -ido:

• comer → comido
• vivir → vivido

So he estudiado literally follows the pattern:

• he = I have
• estudiado = studied

Usually yes. Biblioteca means library.

So:

• en la biblioteca = in/at the library

Just be careful not to confuse it with librería, which means bookstore, not library.

This is a very common learner mix-up:

• biblioteca = library
• librería = bookstore

Put no before the verb phrase:

• Hoy no he estudiado bastante en la biblioteca.

That means something like Today I haven’t studied enough at the library or Today I haven’t studied much at the library, depending on context.

More naturally, if you want to say I haven’t studied much, Spanish often uses:

• Hoy no he estudiado mucho en la biblioteca.

So while no he estudiado bastante is possible, no he estudiado mucho may sound more natural in many situations.

In spoken Spanish, you usually just use intonation:

• ¿Hoy has estudiado bastante en la biblioteca?

That means Have you studied quite a lot at the library today?

Notice that the verb changes because now the subject is you:

• he estudiado = I have studied
• has estudiado = you have studied

Spanish does not usually need do/does/did to make questions.

A rough pronunciation guide is:

oy eh es-too-dee-AH-doh bas-TAHN-teh en la by-blee-oh-TEH-kah

A few important points:

• hoy sounds roughly like oy
• he sounds like eh
• the h in he is silent
• bastante has stress on tan
• biblioteca has stress on te

A more connected pronunciation would sound smoother than the word-by-word guide, but those stress points are useful.