No es justo juzgar a otras personas sin conocer su historia.

Justo is masculine singular because here it’s used in an impersonal structure: No es justo + infinitive (= It’s not fair to…).

There is no specific noun being described; the adjective refers to the idea of juzgar a otras personas sin conocer su historia as a whole. In Spanish, when there is no clear noun, the default is usually masculine singular:

• Es bueno estudiar. – It’s good to study.
• No es fácil aprender chino. – It’s not easy to learn Chinese.

You would use justa only if you had a feminine noun:

• La decisión no es justa. – The decision is not fair.

After es + adjective, when we’re talking about an action in general, Spanish usually uses the infinitive to express that action as the subject:

• Es difícil aprender español. – Learning Spanish is difficult.
• No es bueno fumar. – It’s not good to smoke.

So No es justo juzgar… literally is “To judge… is not fair,” which in natural English becomes It’s not fair to judge…

Because of the “personal a”. In Spanish, when the direct object is a person (or people), you normally add a before it:

• Veo a María. – I see María.
• Ayudamos a los niños. – We help the children.

Here, otras personas are people, so you say juzgar a otras personas. Omitting a (juzgar otras personas) sounds ungrammatical in standard Spanish.

Because the noun persona is grammatically feminine in Spanish, regardless of the actual gender of the person:

• Juan es una persona muy amable. – Juan is a very kind person.

Adjectives and determiners must agree with persona, so you get:

• una persona interesante
• estas personas simpáticas
• otras personas

Even if the group includes only men, you still say personas (feminine) and therefore otras.

In Spanish, su can mean his / her / its / your (formal) / their, depending on context.

Here, su historia is understood as “each person’s story,” like saying in English:

• …without knowing their story (each person’s story).

We keep historia singular because we’re talking about the story of each individual, in a generic way.

You could say sin conocer sus historias, but that emphasizes that there are clearly many different stories, and it’s less common in this kind of general moral statement. Su historia is more idiomatic.

Yes, it’s grammatically correct:

• No es justo juzgar a otras personas sin conocer sus historias.

This sounds more like you’re strongly highlighting that there are many distinct stories. The original su historia feels more general and proverbial: each person has a story, and you don’t know it. Both are possible; the given version is more typical in a moral/general statement.

Conocer is used for:

• Being familiar with people, places, or things on a more personal level.
• Having experience of something.

Saber is for:

• Knowing facts, data, information.
• Knowing how to do something (saber + infinitive).

Here, su historia means something like their life story / personal background. You’re talking about truly being acquainted with that story, not just “knowing facts” about it.

So:

• conocer su historia – to know/understand their story, be familiar with it.
• saber su historia – can work, but sounds more like “to know the facts of their story,” e.g., for a test. In this moral context, conocer is better.

In Spanish, historia can mean both:

• history (as in the past events of something or someone)
• story (a narrative)

In this sentence, su historia is best understood as their (life) story / their background:

• No es justo juzgar a otras personas sin conocer su historia.
  ≈ It’s not fair to judge other people without knowing their story / background.

It’s not about “History” as a school subject; it’s about what each person has lived through.

You can move phrases around in Spanish, but not all orders sound natural.

The most natural versions are:

• No es justo juzgar a otras personas sin conocer su historia.
• No es justo, sin conocer su historia, juzgar a otras personas. (more formal/emphatic)

Your version …sin conocer su historia a otras personas is technically understandable, but it sounds awkward and unnatural. Usually, you keep a otras personas right after juzgar, because it’s directly linked to the verb.

Yes, they’re close but not identical:

• otras personas = other people (very general)
• los demás = the others / everyone else / the rest (of the people)

You could say:

• No es justo juzgar a los demás sin conocer su historia.

This focuses more on “everyone else apart from me/us,” while otras personas is just “other people” in general. Both are fine here; it’s a nuance of emphasis.

In Spanish, to negate a verb, you normally put no directly before the conjugated verb:

• No es justo. – It’s not fair.
• No quiero ir. – I don’t want to go.

You don’t say es no justo in this kind of sentence. The basic pattern is:

no + verb (+ rest of the sentence)

So No es justo juzgar a otras personas… is the standard negative structure.

Yes, that’s grammatically correct:

• El juzgar a otras personas sin conocer su historia no es justo.

Here, el + infinitivo (el juzgar) works like a noun phrase: The judging of other people without knowing their story is not fair.

However, in modern spoken Spanish, the version without el is more natural and less stiff:

• No es justo juzgar a otras personas sin conocer su historia.

The meaning is the same; the original is just more common in everyday language.