Несчастный ребёнок плачет.

Because it is agreeing with ребёнок, which is a masculine singular noun in the nominative case.

In Russian, adjectives change their endings to match the noun’s:

• gender
• number
• case

Here:

• ребёнок = masculine, singular, nominative
• so the adjective takes the masculine singular nominative form: несчастный

If the noun changed, the adjective would change too:

• несчастная девочка = an unhappy girl
• несчастные дети = unhappy children

Here ребёнок is the subject of the sentence, so it is in the nominative case, which is the dictionary form.

Russian uses cases to show a noun’s role in the sentence. Since the unhappy child is the one doing the action of crying, it stays in nominative:

• Несчастный ребёнок плачет.

If it were an object or used after certain prepositions, the form could change:

• Я вижу ребёнка. = I see the child.
  Here ребёнка is accusative.

In the present tense, Russian usually does not use a verb meaning to be in simple sentences like this.

So:

• Несчастный ребёнок плачет. literally looks like:
• Unhappy child cries.

But in natural English, we translate it as:

• The unhappy child is crying or The unhappy child cries, depending on context.

This is completely normal in Russian. The present-tense is/am/are is usually omitted.

Несчастный can mean several related things, including:

• unhappy
• miserable
• unfortunate
• poor in the emotional sense, as in poor child

Which English word fits best depends on context.

In this sentence, because the child is crying, несчастный probably suggests something like:

• unhappy
• miserable
• poor unhappy child

So it is broader than just one exact English word.

In Russian, adjectives often come before the noun, just like in English:

• несчастный ребёнок = unhappy child

That is the most neutral and common order.

Russian word order is more flexible than English, so other orders are sometimes possible for emphasis or style, but adjective + noun is the basic pattern here.

Плачет is the 3rd person singular present-tense form of the verb плакать, meaning to cry.

So:

• я плачу = I cry / I am crying
• ты плачешь = you cry / you are crying
• он / она плачет = he / she cries / is crying

Since ребёнок is a singular subject, the verb is singular too:

• ребёнок плачет

Russian present tense often covers both meanings:

• cries
• is crying

The exact English translation depends on context.

So плачет can mean:

• The child cries if you mean this as a general fact or habit
• The child is crying if you mean it is happening right now

Without more context, both are grammatically possible. In this sentence, many learners would naturally understand it as is crying.

The correct spelling is ребёнок, with ё.

The letter ё shows that the syllable is pronounced with yo:

• re-BYO-nok

In many Russian texts, especially informal or older ones, ё is often replaced by е, so you may also see:

• ребенок

But the pronunciation and the actual word are still ребёнок. For learners, it is very helpful to pay attention to ё, because it affects both pronunciation and stress.

The stress is:

• несча́стный
• ребёнок
• пла́чет

So the sentence is pronounced approximately like:

• ne-SHCHAST-nyy re-BYO-nok PLA-chet

Stress in Russian is very important because it is not always predictable, so it is good to learn each word together with its stress.

Not necessarily. Ребёнок is a grammatically masculine noun, but it can refer to any child, not only a boy.

So even if the child is a girl, Russian can still say:

• ребёнок плачет

Because grammatical gender and real-life biological sex do not always match perfectly.

If you specifically want to say girl, you would use девочка, which is feminine:

• Несчастная девочка плачет.

Because ребёнок has an irregular plural:

• singular: ребёнок
• plural: дети = children

This is one of the suppletive patterns in Russian, where the singular and plural come from different roots, much like English person / people.

So:

• Несчастный ребёнок плачет. = The unhappy child is crying.
• Несчастные дети плачут. = The unhappy children are crying.

Yes. It is a perfectly complete Russian sentence.

It has:

• a subject: несчастный ребёнок
• a verb: плачет

Russian often expresses ideas very compactly, and no extra article such as a or the is needed, because Russian has no articles.

So a short sentence like this is fully natural and complete.