Learn Hardcore Spanish (Latin America): Creo que la justicia es importante en todo el mundo. - I believe that justice is important all over the world.

Correct:
Creo que la justicia es importante.
“I think / I believe that justice is important.”
Incorrect:
Creo la justicia es importante. ❌ (sounds wrong to a native speaker)

Compare:

Creer que + sentence (clause):
Creo que tienes razón. – “I think (that) you’re right.”
Creer en + noun:
Creo en la justicia. – “I believe in justice.”

So:

Creo que + la justicia es importante = “I think that justice is important.”
Creo en la justicia = “I believe in justice (as a value / principle).”

Can I use pienso instead of creo here? Is there a difference?

You can. Both are fine here:

Creo que la justicia es importante...
Pienso que la justicia es importante...

In many everyday contexts, creer and pensar overlap and both mean “to think” (as in “to have an opinion”). Nuance:

creer que – slightly more like “I believe that…”, often opinion, belief, personal conviction.
pensar que – more like “I think that…”, sometimes feels a bit more rational or reflective, but in practice they are often interchangeable.

In your sentence, using creo or pienso doesn’t change the meaning in any important way.

Why is it la justicia and not just justicia, like in English “justice”?

Spanish uses the definite article (el, la, los, las) with general or abstract nouns much more than English does.

La justicia es importante.
Literally: “The justice is important.”
Natural English: “Justice is important.”

Other similar examples:

La libertad es esencial. – “Freedom is essential.”
La educación es cara. – “Education is expensive.”

So:

In Spanish, you usually say la justicia when talking about justice as a general concept.
In English, you normally drop “the” and just say “justice”.

That’s why la justicia is used here even though English has no article.

Why do we use es importante and not está importante?

This is the ser vs. estar contrast.

ser (here: es) is used for inherent, defining, or generally true characteristics.
estar (here: está) is used for temporary states, locations, or conditions.

Es importante here means “is (by nature / generally) important”:

La justicia es importante.
Justice is (in general, as a principle) important.

If you said está importante, it would sound wrong or at best very unusual. Importante with estar is rare and usually colloquial about people:

Hoy está muy importante. (colloquial about a person)
“He/she is acting all important today.”

So for “X is important” in the general, timeless sense, use ser:
X es importante.

Why is it es importante (indicative) and not sea importante (subjunctive) after creo que?

After creo que in an affirmative statement, Spanish normally uses the indicative because the speaker presents the idea as something they believe to be true:

Creo que la justicia es importante. ✅
“I think justice is important.”

You use the subjunctive (sea) after no creo que or other expressions of doubt/negation:

No creo que la justicia sea importante. ✅
“I don’t think justice is important.”
Dudo que la justicia sea importante. ✅
“I doubt that justice is important.”

So:

Affirmative creer que → usually indicative: es
Negative / doubtful (no creer que, dudar que, etc.) → subjunctive: sea

Is it okay to say Yo creo que la justicia es importante or should I drop yo?

Both are grammatically correct:

Creo que la justicia es importante.
Yo creo que la justicia es importante.

In Spanish, the subject pronoun (yo, tú, él, etc.) is often omitted because the verb ending already shows the person:

creo = I think
crees = you think
cree = he/she/you (formal) think(s)

You usually add yo for:

Emphasis or contrast:
Yo creo que la justicia es importante, pero él no.
“I think justice is important, but he doesn’t.”
Clarity, when context might be ambiguous.

In a neutral sentence like yours, Creo que… (without yo) sounds the most natural, but Yo creo que… is perfectly fine.

Why is it en todo el mundo and not por todo el mundo?

Both phrases exist, but they’re used a bit differently.

en todo el mundo – literally “in the whole world”; focuses on the idea “in every country / place”.
por todo el mundo – literally “throughout / all over the world”; often used with movement, spreading, distribution.

Your sentence:

Creo que la justicia es importante en todo el mundo.
“I think justice is important in every part of the world.”

If you said:

La noticia se difundió por todo el mundo.
“The news spread all over the world.”

Here por fits because something is moving/spreading across places.

You could hear importante por todo el mundo, but importante en todo el mundo is the standard and feels more natural when you mean “it is important in all countries.”

Can I move en todo el mundo to another place in the sentence?

Yes. Spanish word order is somewhat flexible, especially for phrases like this. All of these are grammatically correct, with only slight changes in emphasis:

Creo que la justicia es importante en todo el mundo.
(Neutral; most common.)
Creo que en todo el mundo la justicia es importante.
(Slight extra focus on “in all the world”.)
En todo el mundo, creo que la justicia es importante.
(Starts by setting the context “in the whole world”.)

Meaning stays basically the same; you just highlight different parts of the sentence.

Does todo el mundo also mean “everyone”? How do I know it means “the whole world” here?

Yes, todo el mundo has two common meanings in Spanish:

“Everyone” / “everybody”
- Todo el mundo está aquí.
  “Everyone is here.”
“The whole world” / “the entire world”
- Hay problemas en todo el mundo.
  “There are problems all over the world.”

In your sentence:

Creo que la justicia es importante en todo el mundo.

This clearly means “in the whole world”, because en normally introduces a place (“in/at”), and la justicia es importante en todo el mundo is about where it is important (everywhere on Earth), not about people.

If you wanted to say “for everyone”, you would usually change the wording:

Creo que la justicia es importante para todo el mundo.
“I think justice is important for everyone.”

So:

en todo el mundo → place (“in the whole world”)
para todo el mundo (or para todos) → people (“for everyone”)

How would I say “I don’t think justice is important in the whole world” in Spanish?

You have two main options, and they use different moods:

No creo que + subjunctive
Focus on doubting/denying the idea itself:
- No creo que la justicia sea importante en todo el mundo.
  “I don’t think justice is important in the whole world.”
Creo que + negation in the clause (with indicative)
Slightly shifts emphasis; often sounds like a more reasoned opinion:
- Creo que la justicia no es importante en todo el mundo.
  “I think justice is not important in the whole world.”

Both are valid, but:

With no creo que, use subjunctive: sea.
With creo que … no, use indicative: no es.

How would the sentence change if I talked about plural things like “laws” instead of “justice”? What happens to the verb and adjective?

If you make the subject plural, the article, noun, verb, and adjective all need to agree in number (singular/plural).

Original (singular):

Creo que la justicia es importante en todo el mundo.

With a plural noun, e.g. “laws” (las leyes):

Creo que las leyes son importantes en todo el mundo.
- las leyes – plural noun
- son – plural of es
- importantes – plural of importante

So the pattern is:

la justicia es importante → singular
las leyes son importantes → plural

About justicia itself:

justicia is feminine singular (because of its form and meaning; many -cia nouns are feminine).
It’s usually used as an uncountable abstract noun, so you almost always see la justicia, not las justicias.
In specialized legal contexts you might see las justicias meaning “the different justice systems/courts”, but that’s uncommon in everyday language.