هذا الطفل صغير.

In Arabic, the present tense of to be is usually not expressed in simple sentences like this.

So:

• هذا الطفل صغير = This child is small
• literally, something like This child small

This kind of sentence is called a nominal sentence in Arabic. If you wanted was or will be, Arabic would use other forms, but in the simple present, there is usually no separate word for is.

هذا is the masculine singular demonstrative meaning this.

It comes before the noun it points to:

• هذا الطفل = this child

So the sentence begins with the thing being talked about: this child.

A useful comparison:

• هذا الطفل = this child
• هذه الطفلة = this girl / this female child

So هذا must match a masculine singular noun.

This is a very common question for English speakers.

In Arabic, after a demonstrative like هذا (this), the noun is usually definite, so it normally takes الـ.

So:

• هذا الطفل = this child

But:

• هذا طفلٌ usually means This is a child

Those are different structures:

• هذا الطفل = demonstrative + definite noun = this child
• هذا طفلٌ = this
  • predicate noun = this is a child

So in your sentence, الطفل is definite because Arabic normally says this the-child, where English says simply this child.

Because here صغير is the predicate of the sentence, not an adjective inside the noun phrase.

Compare these two:

• هذا الطفل صغير = This child is small
• هذا الطفل الصغير = this small child

In the first sentence:

• هذا الطفل = the subject phrase
• صغير = what is being said about the child

In the second phrase:

• الصغير directly describes الطفل
• both are definite, so both take الـ

So:

• الطفل صغير = the child is small
• الطفل الصغير = the small child

That difference is extremely important in Arabic.

It is originally an adjective, but in this sentence it functions as the predicate.

So grammatically, it is an adjective in form, but its job in the sentence is to say something about the subject:

• الطفل صغير = the child is small

This is very common in Arabic. An adjective can be used as the predicate of a nominal sentence.

So a simple way to think about it is:

• form: adjective
• function here: predicate

A careful full pronunciation is:

• hādhā aṭ-ṭiflu ṣaghīrun

A pause-form pronunciation, which is very common in actual reading and speech, is:

• hādhā aṭ-ṭifl ṣaghīr

Why aṭ-ṭifl and not al-ṭifl?

Because ط is a sun letter, so the ل of الـ is not pronounced separately. Instead, the following consonant is doubled in pronunciation.

So:

• written: الطفل
• pronounced: aṭ-ṭifl

This happens with other sun letters too.

With full case endings, the sentence is:

• هذا الطفلُ صغيرٌ

Here:

• الطفلُ ends in ـُ because it is in the nominative
• صغيرٌ also ends in ـٌ because the predicate of a nominal sentence is also nominative

A more advanced note: هذا itself is generally treated as indeclinable, so it does not show a changing case ending the way many nouns do.

In normal printed Arabic, these short vowels are often omitted, so you usually just see:

• هذا الطفل صغير

You would change both the demonstrative and the predicate to match the feminine noun.

For example:

• هذه الطفلة صغيرة = This girl / this female child is small

Changes:

• هذا → هذه for feminine singular
• الطفل → الطفلة
• صغير → صغيرة

So the predicate agrees with the noun in gender.

A simple learner-friendly breakdown is:

• هذا الطفل = the subject phrase = this child
• صغير = the predicate = small

A slightly more traditional grammar explanation is:

• هذا = the main topic/subject element
• الطفل = a clarifying noun attached to هذا
• صغير = the predicate (خبر)

If you are a beginner, the easiest way to understand it is just:

• هذا الطفل = who we are talking about
• صغير = what we are saying about him

They look similar, but they mean different things.

• هذا الطفل صغير = This child is small
• الطفل الصغير = the small child

The first is a complete sentence. The second is just a noun phrase.

A good shortcut:

• if the second word describes the noun and both are definite, it is usually a noun phrase: الطفل الصغير
• if the final adjective is indefinite and tells you something about the subject, it is often the predicate: الطفل صغير

So صغير without الـ helps signal that this is a sentence meaning is small, not just small child.