kono kodomo ha kasikoi.

この is a demonstrative word meaning this, and it must directly modify a noun.

So:

• この子供 = this child
• この cannot stand alone by itself here

If you want this one by itself, you would use これ instead.

Compare:

• この子供 = this child
• これ = this one

は marks the topic of the sentence.

In この子供は賢い, the topic is この子供 (this child), and the sentence says something about that topic: 賢い (smart / clever).

A natural way to understand the structure is:

• As for this child, [he/she] is smart.

So は often feels like as for... or sometimes like the English subject marker, but it is specifically a topic marker.

When は is used as a particle, it is pronounced wa, not ha.

So this sentence is read:

• この こども は かしこい
• pronunciation: kono kodomo wa kashikoi

But when は is part of a regular word, it is usually pronounced ha.

This is one of the first special pronunciation rules learners encounter in Japanese.

Japanese often does not need a separate word like English is.

In this sentence, 賢い is an i-adjective, and i-adjectives can act as the predicate by themselves.

So:

• この子供は賢い。
• literally: As for this child, smart.
• natural English: This child is smart.

You do not need a verb meaning is here.

賢い is an i-adjective.

You can tell because it ends in い in its dictionary form. i-adjectives can:

• directly modify nouns
  
  • 賢い子供 = a smart child
• end a sentence by themselves
  
  • この子供は賢い = this child is smart

This is different from na-adjectives, which need だ or です when used at the end of a sentence.

Yes, but the nuance changes.

• この子供は賢い = This child is smart / As for this child, he/she is smart
• この子供が賢い puts stronger focus on this child as the one who is smart

Using が can sound like:

• This child is the smart one
• It is this child who is smart

So は is more neutral for a simple statement, while が often adds focus or contrast.

Yes. In this sentence, 子供 is read こども (kodomo).

A couple of useful notes:

• 子供 means child / children, depending on context
• Japanese nouns usually do not change form for singular vs. plural

So 子供 can mean:

• a child
• children

In this sentence, because of この (this), it is clearly singular: this child.

You may also see it written as 子ども, which is very common.

Because Japanese sentence order is different from English.

The basic idea here is:

• topic + comment

So:

• この子供は = as for this child
• 賢い = smart

Japanese often puts the main statement at the end. In English, we say This child is smart, but in Japanese the descriptive word can come last without needing is.

Not exactly.

賢い usually means:

• clever
• smart
• wise
• bright

It often suggests good judgment or intelligence. Depending on context, it can sound a little more like clever or wise than just academically intelligent.

For pure academic ability, people also often use:

• 頭がいい = smart / intelligent

But 賢い is completely natural here.

Often, yes.

• この子供は賢い。 is plain style
• この子供は賢いです。 is polite style

The plain version is fine in casual conversation, narration, or informal writing.
The です version is better when speaking politely.

So if you are talking to a teacher, stranger, or someone you should be polite with, この子供は賢いです is usually safer.

You are right: normal Japanese writing usually does not use spaces.

The standard written form is:

• この子供は賢い。

Spaces are often added in beginner materials to make the parts easier to read:

• この / 子供 / は / 賢い

So the spaced version is just a learning aid, not standard Japanese formatting.

Because 賢い is an i-adjective, it changes form.

• この子供は賢くない。 = This child is not smart.
• この子供は賢かった。 = This child was smart.
• この子供は賢くなかった。 = This child was not smart.

Polite forms:

• この子供は賢くないです / 賢くありません
• この子供は賢かったです
• この子供は賢くなかったです / 賢くありませんでした

This is a useful pattern for all i-adjectives.