Yo prefiero la comedia, pero mi madre quiere ver el documental primero.

Yes. Spanish often drops subject pronouns because the verb ending already shows who is doing the action.

• Yo prefiero la comedia = I prefer comedy
• Prefiero la comedia = also I prefer comedy

Here, yo is probably included for emphasis or contrast, especially because the sentence compares two people:

• Yo prefiero la comedia, pero mi madre quiere ver el documental primero.

So the yo helps highlight my preference vs. my mother’s preference.

Because preferir is a stem-changing verb.

In the present tense, the e in the stem changes to ie in most forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So prefiero is the correct yo form.

This is very common in Spanish. Other verbs do the same kind of change, such as:

• querer → quiero
• pensar → pienso

Spanish nouns have grammatical gender, so you usually need the correct article:

• la comedia → feminine
• el documental → masculine

In this sentence:

• comedia is feminine, so it takes la
• documental is masculine, so it takes el

There is not always a perfect logical reason, so gender often has to be learned with the noun.

A useful habit is to learn nouns with their article:

• la comedia
• el documental
• la película
• el cine

Spanish often uses the definite article with nouns when talking about things in a general sense.

So:

• Prefiero la comedia literally looks like I prefer the comedy
• but it really means I prefer comedy / I prefer comedies

This is very normal in Spanish. You also see it in sentences like:

• Me gusta la música = I like music
• Odio el ruido = I hate noise

So even though English often leaves the article out, Spanish often keeps it.

Because Spanish possessive adjectives usually replace the article.

So you say:

• mi madre = my mother
• tu padre = your father
• su hermana = his/her your/their sister

Not:

• la mi madre ✗

This is different from English only in structure, but it works consistently in Spanish.

Also note that mi does not change for masculine or feminine:

• mi madre
• mi padre

Because after querer in this kind of sentence, Spanish uses an infinitive directly.

• quiere ver = wants to watch / wants to see
• quiero comer = I want to eat
• queremos salir = we want to go out

English uses to before the second verb, but Spanish usually does not.

So:

• mi madre quiere ver el documental
• literally: my mother wants watch the documentary
• natural English: my mother wants to watch the documentary

In this sentence, ver is best understood as to watch.

Spanish ver can cover both ideas depending on context:

• ver una película / un documental / la televisión = to watch
• ver algo = to see something

Since a documental is something you watch, ver naturally means watch here.

Here primero means first.

So:

• mi madre quiere ver el documental primero = my mother wants to watch the documentary first

It comes at the end because that is a very natural place for it in Spanish. It modifies the action of watching.

You can also hear:

• Mi madre quiere ver primero el documental

That is also correct. The difference is mostly about emphasis and rhythm, not basic meaning.

In this sentence, putting primero at the end sounds very natural and clear.

No, not in this sentence.

Here primero is being used as an adverb, meaning first.

• ver el documental primero = watch the documentary first

If you wanted the first documentary, primero would act more like an adjective and would go with the noun:

• el primer documental = the first documentary

Notice that before a masculine singular noun, primero usually shortens to primer:

• el primer libro
• el primer documental

So in your sentence, primero clearly means first, not first documentary.

Because el documental is a thing, not a person.

Spanish uses the personal a mainly before specific people (and sometimes pets or personified animals) when they are direct objects.

Compare:

• Quiere ver el documental = correct, no a
• Quiere ver a su madre = correct, because su madre is a person

So there is no a here because the documentary is inanimate.

Because pero means but, and in Spanish it is very common to put a comma before it when it joins two full ideas.

Here the sentence has two parts:

• Yo prefiero la comedia
• pero mi madre quiere ver el documental primero

The comma helps separate the contrast clearly. This is similar to English, where we also often write:

• I prefer comedy, but my mother wants to watch the documentary first.

Yes, documental can be a noun as well as an adjective.

As a noun:

• el documental = the documentary

As an adjective:

• una película documental = a documentary film
• material documental = documentary material

In everyday Spanish, documental is very commonly used as a noun on its own.

This sentence is completely natural in Spain and also widely natural in Latin America. Nothing in it is strongly region-specific.

A few notes:

• comedia
• madre
• quiere ver
• documental
• primero

are all standard everywhere.

The main thing that might differ by region is pronunciation, not grammar. For example, in most of Spain, c in comedia is pronounced like an English th before e/i only if it were ce/ci, but here it is just a normal k sound because it is co. So this sentence doesn’t really contain one of the classic Spain-vs-Latin-America pronunciation differences in a major way.

Grammatically, the sentence works the same across the Spanish-speaking world.