الأمن مهم.

A careful MSA pronunciation with full case endings is:

al-amnu muhimmun

In normal pause, learners often hear it more like:

al-amn muhimm

A few notes:

• الأمن = al-amn(u)
• مهم = muhimm(un)
• The final short vowels are often not pronounced in everyday reading aloud unless someone is reading very formally.

Because Arabic usually does not use a present-tense verb to be in simple sentences like this.

So:

• الأمن مهم literally looks like security important
• but it means Security is important

This is completely normal in Arabic.

If you want past or future, then Arabic does use a verb:

• كان الأمن مهمًا = Security was important
• سيكون الأمن مهمًا = Security will be important

It is a complete sentence.

In Arabic grammar, this is a nominal sentence:

• الأمن = the topic / subject-like part
• مهم = the predicate / what is being said about it

So the structure is:

• الأمن = security
• مهم = important

Together: Security is important

Even though مهم is an adjective in form, here it functions as the predicate of the sentence.

الـ is the Arabic definite article, usually equivalent to the.

So:

• أمن = security, safety
• الأمن = the security

However, Arabic often uses the definite article where English uses no article, especially with general ideas and abstract nouns.

That is why:

• الأمن مهم naturally means Security is important

not necessarily only The security is important

This is a very common pattern in Arabic.

Because in this sentence مهم is the predicate, not an adjective attached directly to the noun.

Compare:

• الأمن مهم = Security is important
  This is a sentence.

But:

• الأمن المهم = the important security
  This is just a noun phrase, not a full sentence.

So leaving مهم without الـ helps show that it is the predicate and that the sentence is complete.

With full case endings, the sentence is:

الأَمْنُ مُهِمٌّ

That is pronounced:

al-amnu muhimmun

Why those endings?

• الأمنُ is the mubtadaʼ (the opening noun in a nominal sentence), so it is nominative
• مهمٌّ is the khabar (predicate), so it is also nominative

In ordinary unvowelled writing, these endings are usually not shown.

Because it matches الأمن, which is grammatically masculine singular.

In Arabic, predicates and adjectives often agree in gender and number with the noun they describe or refer to.

So:

• الأمن مهم = masculine singular
• If the noun were feminine, you would use مهمة

For example:

• السلامة مهمة = Safety is important

It can mean either, depending on context.

Common possibilities include:

• security
• safety
• public security
• stability/order, in some contexts

So الأمن مهم could be understood as:

• Security is important
• Safety is important

The exact English choice depends on the situation.

Because the basic word is أمن, which begins with a hamza.

When you add the definite article الـ, the original hamza of the word stays:

• أمن
• الأمن

So the spelling reflects:

• the definite article الـ
• plus the original word أمن

This is why the word is not written as just المن.

Because مهم ends with a shadda on the final م in its fully vowelled form:

مُهِمٌّ

That means the last m sound is doubled or held slightly longer:

• mu-himm

This is part of the normal form of the word مهم meaning important.

So when pronouncing it, try not to say muhim only once; say it more like muhimm.

Grammatically, you could form something like that, but it would usually not mean the same thing.

• الأمن مهم = Security is important
  This is the natural general statement.

But:

• أمن مهم would sound more like an important security/safety, which is not usually what you want.

So for the general idea Security is important, الأمن مهم is the natural choice in MSA.