El niño es inocente.

In Spanish, a singular countable noun used as the subject of a sentence normally needs a determiner, such as an article.

So:

• El niño es inocente. = The boy is innocent.
• Un niño es inocente. = A boy is innocent.

Just Niño es inocente is not normally correct as a complete sentence.

Spanish uses articles more often than English in some situations, so keeping el here is very natural.

Because inocente is being treated as an essential quality, identity-related description, or status, so Spanish uses ser:

• El niño es inocente.

Use ser for things like:

• identity
• profession
• origin
• basic characteristics
• classifications

Estar is usually for states, conditions, or locations.

With inocente, Spanish normally uses ser when you mean innocent in the sense of not guilty or naturally innocent.

So:

• es inocente = he is innocent

Using está inocente would sound unnatural in standard usage.

In Spanish, most descriptive adjectives usually come after the noun.

So:

• el niño inocente
• el coche rojo
• la casa grande

That is why inocente naturally comes after niño in a sentence like this.

You can sometimes put an adjective before the noun, but that often changes the tone, emphasis, or style:

• el inocente niño sounds more literary, emotional, or emphatic

The neutral, everyday order is:

• el niño inocente
• and in your full sentence: El niño es inocente.

Not in the singular.

Inocente is an adjective with the same form for both masculine and feminine singular:

• El niño es inocente.
• La niña es inocente.

In the plural, it becomes:

• Los niños son inocentes.
• Las niñas son inocentes.

So the only change is usually number:

• singular: inocente
• plural: inocentes

Because the subject is already stated: el niño.

Spanish does not need an extra subject pronoun when the subject is already clear.

So:

• El niño es inocente. = perfectly normal

Adding él would usually be unnecessary or emphatic:

• Él es inocente. = He is innocent.
• El niño es inocente. = The boy is innocent.

You use one or the other depending on what you want as the subject.

Niño is pronounced roughly like NEEN-yo, but with the Spanish ñ sound.

Important points:

• ñ is like the ny sound in canyon
• niño is approximately NEE-nyo
• the stress is on the first syllable: NI-ño

Also, ñ is not just n with an accent. It is a separate letter in Spanish.

So:

• n and ñ are different sounds
• niño and nino would not be the same

Not exactly.

The mark over ñ is called a tilde, but in this case it is part of the letter itself. Ñ is a separate Spanish letter, not just an n with stress marked.

So in niño:

• there is no written stress accent
• the tilde is part of ñ

Stress is still regular:

• niño ends in a vowel, so Spanish naturally stresses the second-to-last syllable
• that gives NI-ño

Usually not in the same way.

• El niño es inocente. refers to a specific boy: the boy
• Un niño es inocente. can sound odd as a simple statement, unless the context specifically introduces a boy

For example:

• Hay un niño en la sala. El niño es inocente. = There is a boy in the room. The boy is innocent.
• Un niño es inocente by itself sounds less natural unless you mean something more general or are setting up a story

So if you mean a specific boy, el niño is the normal choice.

Because niño refers to a boy, so it is a masculine noun.

Spanish nouns have grammatical gender, and words referring to male people are usually masculine:

• el niño = the boy
• la niña = the girl

That affects the article:

• el for masculine singular
• la for feminine singular

But in this sentence, the adjective inocente stays the same:

• El niño es inocente.
• La niña es inocente.

You change the article, noun, verb, and adjective as needed:

• El niño es inocente. = singular
• Los niños son inocentes. = plural

Changes:

• el → los
• niño → niños
• es → son
• inocente → inocentes

If feminine:

• La niña es inocente.
• Las niñas son inocentes.

Yes, it can.

Depending on context, inocente can mean:

• innocent in a moral or emotional sense
• naive or pure
• not guilty of a crime or accusation

So El niño es inocente could mean:

• the boy has done nothing wrong
• the boy is blameless
• the boy is innocent in a general sense

The exact nuance depends on context.