Learn Hardcore Spanish (Latin America): Su mejor amiga es periodista y escribe sobre derechos humanos. - Her best friend is a journalist and writes about human rights.

Only the wider context tells you whose best friend it is. Spanish does not specify gender or number of the owner in the word su itself; it just agrees in number with the noun it modifies (su amiga / sus amigas).

Why is it “mejor amiga” and not “amiga mejor”?

In Spanish, most adjectives go after the noun, but some very common, short adjectives often go before the noun, especially when they express a more subjective or evaluative quality.

Mejor (better / best) is one of those. So:

mi mejor amiga = my best friend
mi amiga mejor sounds odd or wrong in this context.

Use mejor before the noun for “best”:

mi mejor amigo, su mejor profesor, su mejor momento, etc.

Why is it “es periodista” and not “es una periodista”?

With professions, religions, and nationalities, Spanish normally drops the indefinite article (un/una) when it simply states someone’s occupation or identity:

Es periodista. = She is a journalist.
Es médico. = He is a doctor.
Es mexicana. = She is Mexican.

You generally add the article when you qualify it with an adjective or a phrase that makes it more specific:

Es una periodista muy famosa. = She is a very famous journalist.
Es un médico excelente. = He is an excellent doctor.

So in this neutral statement, “es periodista” is the standard form.

Why is it “es periodista” and not “está periodista”? What’s the difference?

Spanish uses ser for more permanent or defining characteristics, and estar for states/conditions that are more temporary or situational.

Ser
- profession = the person’s role/identity.
- Es periodista. → Being a journalist is what she is (profession).
Estar is not used with professions in this simple way.
- Está periodista is incorrect.

You could use estar with a profession only in special structures, e.g.:

Está de periodista. = She’s working as a journalist (for now / in a temporary role).

But the neutral “She is a journalist” is always es periodista.

Is “periodista” feminine because of “amiga”? What if the journalist is male?

The word periodista has the same form for both genders. The gender is shown by the article or related words, not by changing the ending:

la periodista → female journalist
el periodista → male journalist

In this sentence, we know the friend is female (amiga), so es periodista refers to a female journalist.

If you changed the sentence to a male friend:

Su mejor amigo es periodista. → His/Her/Your/Their best (male) friend is a journalist.

The word periodista itself stays the same.

Why is it “escribe” (simple present) and not something like “está escribiendo”?

Spanish uses the simple present much more often than English to talk about:

habits or regular actions
general truths
ongoing work or activities in someone’s life

So:

escribe sobre derechos humanos = she writes about human rights (as what she generally does, her beat or specialty).

You would use está escribiendo only if you really mean “she is writing *right now”*:

Ahora está escribiendo sobre derechos humanos. = Right now she is writing about human rights.

Here, the sentence describes what she usually writes about, so escribe is the natural tense.

What does “sobre” mean in “escribe sobre derechos humanos”? Is it like “about” or “on”?

In this context, sobre means “about” or “on (the topic of)”:

escribe sobre derechos humanos
- she writes about human rights
- she writes on human rights

Other possible translations of sobre (in other contexts) include “on top of” or “over”, but with writing, talking, reading, etc., sobre is usually “about / on”:

un libro sobre música = a book about music
un artículo sobre política = an article on politics

Could you also say “escribe acerca de derechos humanos” or “escribe de derechos humanos”?

Yes, but the nuances differ slightly:

escribe sobre derechos humanos
- Most common and neutral: “writes about human rights.”
escribe acerca de los derechos humanos
- Also correct and a bit more formal/literary: “writes about human rights.”
- Often used in essays, academic writing, etc.
escribe de derechos humanos
- Possible, but less common with “escribir” in this sense.
- You’re more likely to hear hablar de, tratar de, informar de, etc.
  For “write about” specifically, escribir sobre is the most natural in modern usage.

So “escribe sobre derechos humanos” is the best choice here.

Why is it “derechos humanos” (plural)? Could you say “derecho humano”?

Derechos humanos is a fixed expression in Spanish meaning “human rights” as a concept:

los derechos humanos = human rights (in general, as recognized by law, etc.)

You could say derecho humano (singular) if you mean one specific right:

el derecho humano al agua potable = the human right to drinking water

But when you refer to the field or topic in general—like the journalist’s beat—you almost always use the plural derechos humanos.

Why isn’t “derechos humanos” capitalized in Spanish like “Human Rights” sometimes is in English?

Spanish capitalization rules are more conservative than English. In Spanish:

Common nouns and general concepts are not capitalized:
- derechos humanos, derecho civil, derechos laborales

They can be capitalized only when part of an official name or title, e.g.:

Declaración Universal de los Derechos Humanos

In the sentence you gave, derechos humanos is just the general field/topic, not a specific official title, so it stays in lowercase.

Could (or should) you say “sobre los derechos humanos” instead of “sobre derechos humanos”?

Both are possible, but there is a nuance:

escribe sobre derechos humanos
- More general: she writes about human rights as a topic/field.
- Sounds very natural as a description of her beat or specialty.
escribe sobre los derechos humanos
- Feels a bit more specific or “concrete”: she writes about the human rights (maybe those recognized in a particular system), though in practice many speakers use it almost interchangeably.

In everyday speech, without the article is very common for broad subject areas:

escribe sobre política, sobre economía, sobre derechos humanos.

Does “mejor amiga” mean “best female friend,” or could it mean “girlfriend” like in English sometimes?

In Spanish, amiga primarily means female friend, not necessarily a romantic partner.

To specify a romantic relationship, Spanish usually uses:

novia = girlfriend / fiancée
pareja = partner
or you make it clear from context.

So su mejor amiga by itself is normally understood as “his/her/your/their best (female) friend,” not necessarily a girlfriend. Context would need to suggest a romantic meaning if that’s intended.