هذا مستحيل.

It is usually pronounced hādhā mustaḥīl.

• هذا = hādhā
• مستحيل = mustaḥīl

A few pronunciation notes:

• The dh in hādhā sounds like th in this
• The ḥ in mustaḥīl is a stronger, deeper h than the English h
• The ī in mustaḥīl is a long vowel, like ee in see

In Arabic, the verb to be is usually left out in the present tense.

So هذا مستحيل literally looks like this impossible, but in natural English it means this is impossible.

This is very normal in Modern Standard Arabic. If you want a past or future meaning, then Arabic uses a verb:

• كان هذا مستحيلاً = This was impossible
• سيكون هذا مستحيلاً = This will be impossible

It is a full sentence.

Arabic often makes sentences with:

• a subject/topic
• a predicate/comment

Here:

• هذا = the subject, this
• مستحيل = the predicate, impossible

So this is not just a noun followed by an adjective. It is a complete statement: This is impossible.

In traditional Arabic grammar:

• هذا is the مبتدأ (mubtadaʾ) — the thing being talked about
• مستحيل is the خبر (khabar) — what is being said about it

So the structure is very simple:

• هذا = this
• مستحيل = is impossible

هذا is the masculine singular form of this.
هذه is the feminine singular form.

Arabic demonstratives usually agree with the gender of the thing being referred to. So:

• هذا مستحيل = This is impossible for a masculine or general/unspecified referent
• هذه مستحيلة = This is impossible for a feminine referent

For example:

• هذا القرار مستحيل would sound odd semantically, but هذا الأمر مستحيل = This matter is impossible
• هذه الفكرة مستحيلة = This idea is impossible

When English says this without naming a noun, Arabic often uses masculine هذا as a default for a general situation or thing.

Because here مستحيل is the predicate, and predicates are often indefinite in Arabic.

So:

• هذا مستحيل = This is impossible

If you said هذا المستحيل, that would no longer be the same kind of sentence. It would sound more like a noun phrase, such as this impossible one/thing, depending on context.

So the version without ال is the normal one for the meaning This is impossible.

In fully vocalized Modern Standard Arabic, yes: it would be هذا مستحيلٌ.

Why?

Because مستحيل is the predicate of a nominal sentence, and it is in the nominative case.

A few important points:

• In normal Arabic writing, short vowels are usually not written
• In pause, the final case ending is often not pronounced anyway

So learners usually see and say:

• هذا مستحيل

even though the fully grammatical form is:

• هذا مستحيلٌ

It is basically an adjective meaning impossible, but in this sentence it functions as the predicate.

That means it is not directly modifying a noun here. Instead, it completes the statement.

Compare:

• أمرٌ مستحيلٌ = an impossible matter → here مستحيل is an adjective describing أمر
• هذا مستحيلٌ = This is impossible → here مستحيل is the predicate of the sentence

So the word itself is adjective-like, but its job in this sentence is predicative.

Yes. The structure is the same.

• هذا مستحيل = This is impossible
• ذلك مستحيل = That is impossible

The difference is just:

• هذا = this
• ذلك = that

So if the thing is conceptually farther away, or you want that instead of this, you can use ذلك.