هذا أمر مهم.

In Arabic, the verb to be is usually not stated in the present tense.

So هذا أمر مهم literally looks like:

• هذا = this
• أمر مهم = an important matter / an important thing

But the natural English translation is This is an important matter or This is important.

This is called a nominal sentence in Arabic.

The sentence has two main parts:

• هذا = the subject-like part, this
• أمر مهم = the predicate, telling you what this is

More specifically:

• هذا is a demonstrative: this
• أمر is a noun: matter / affair / thing / issue / command
• مهم is an adjective: important

So the structure is:

هذا + أمر + مهم
this + matter + important

Very literally: This [is] an important matter.

Because أمر is a masculine noun in Arabic.

Arabic demonstratives must match the gender of the noun they refer to:

• هذا = this (masculine)
• هذه = this (feminine)

Since أمر is masculine, Arabic uses هذا.

The tanwīn -ٌ shows that أمر is indefinite.

So:

• أمرٌ = a matter / a thing / an affair
• الأمرُ = the matter / the issue / the affair

In this sentence, the meaning is This is an important matter, not This is the important matter, so the noun is indefinite.

The adjective مهم is also indefinite for the same reason, so it appears as مهمٌّ in fully vowelled Arabic.

Because the two expressions mean different things.

• هذا أمر مهم = This is an important matter
• هذا الأمر المهم = this important matter

In هذا أمر مهم, you have a complete sentence:

• هذا = this
• أمر مهم = an important matter

In هذا الأمر المهم, you have a noun phrase, not the same kind of sentence:

• هذا directly points to the important matter

So the indefinite أمر helps show that the sentence means This is an important matter, not This important matter.

In Arabic, adjectives normally come after the noun they describe.

So:

• أمر مهم = an important matter
• literally: matter important

This is the normal Arabic word order for noun + adjective.

Also, the adjective must match the noun in things like:

• gender
• number
• definiteness
• case

So because أمر is:

• masculine
• singular
• indefinite
• nominative

the adjective مهم matches it.

أمر is a very common word with several related meanings, such as:

• matter
• affair
• issue
• thing
• sometimes command/order, depending on context

In هذا أمر مهم, the most natural meaning is usually:

• This is an important matter
• This is an important issue
• This is something important

The exact English translation depends on context.

In fully vowelled, formal Arabic, the sentence is:

هذا أمرٌ مهمٌّ

Both أمرٌ and مهمٌّ are in the nominative here.

Why?

• هذا is the topic/subject part of the sentence
• أمرٌ is the predicate
• مهمٌّ describes أمرٌ, so it matches its case

A learner will often see or hear the sentence without paying much attention to the endings, especially outside very formal reading, but in careful MSA grammar they are nominative.

A common transliteration is:

hādhā amrun muhimmun

In careful formal pronunciation, the endings are heard:

• hādhā
• amrun
• muhimmun

In pause, especially at the end of a sentence, the final short vowels/tanwīn may not be fully pronounced.

Yes. هذا مهم means:

• This is important

That version is shorter and more direct.

The difference is:

• هذا مهم = This is important.
• هذا أمر مهم = This is an important matter / This is something important.

So هذا أمر مهم sounds a bit more explicit because it includes the noun أمر.

A common way is:

هذا ليس أمرًا مهمًّا

This means This is not an important matter.

Notice that after ليس, the predicate becomes accusative:

• أمرًا
• مهمًّا

In simpler learner terms:

• positive: هذا أمرٌ مهمٌّ
• negative: هذا ليس أمرًا مهمًّا

So the structure changes slightly when you use ليس.