القلب مهم.

القلب means the heart.

It is pronounced roughly al-qalb.

A few pronunciation notes:

• الـ = al- = the
• ق is a deep q sound, not an English k
• The word قلب is qalb, with a short vowel after q
• The final consonants cluster together: -lb

So the full word is al-qalb = the heart.

Because الـ is the Arabic definite article, meaning the.

So:

• قلب = a heart / heart
• القلب = the heart

In this sentence, the noun is definite, so Arabic uses الـ.

In Arabic, the verb to be in the present tense is usually not written or spoken in simple sentences like this.

So:

• القلب مهم literally looks like the heart important
• but it means the heart is important

This is completely normal in Arabic.
If you want to talk about the past or future, then forms of كان or other verbs may appear, but in the present tense, Arabic usually leaves is/am/are out.

مهم means important.

In this sentence, it is the predicate adjective, meaning it describes the subject القلب.

So the structure is:

• القلب = the subject
• مهم = important / the predicate

Together: The heart is important.

Because القلب is a masculine singular noun, and adjectives must agree with the noun they describe.

So:

• masculine singular: مهم
• feminine singular: مهمة

Since القلب is masculine, the correct form is مهم.

For comparison:

• القلب مهم = The heart is important.
• القصة مهمة = The story is important.

Yes. In Arabic, adjectives usually come after the noun they describe.

So:

• القلب المهم = the important heart

But in القلب مهم, the adjective is not just modifying the noun inside a noun phrase. It is acting as the predicate of the sentence:

• القلب مهم = The heart is important

So the word order still places the adjective after the noun, but here it is part of a full sentence, not just a noun + adjective phrase.

Yes. القلب مهم is a nominal sentence in Arabic, called a جملة اسمية.

That means it begins with a noun rather than a verb.

The two main parts are:

• المبتدأ: القلب
• الخبر: مهم

So this is a very basic and common Arabic sentence pattern:

• noun + adjective
• meaning X is Y

In full formal Arabic with case endings, yes, it would be:

• القلبُ مهمٌّ

Here:

• القلبُ has ḍamma because it is the subject
• مهمٌّ also has ḍamma/tanwīn because it is the predicate

However, in most learning materials, everyday pronunciation, and unvocalized writing, these endings are often omitted, so you will usually see:

• القلب مهم

Both represent the same sentence; the second is just the normal written form without short vowels.

It can be both, just like in English.

القلب can mean:

• the physical heart
• the emotional or spiritual heart
• the center of feelings

Without context, القلب مهم could mean:

• The heart is important physically
• or The heart is important in an emotional or moral sense

Arabic often uses قلب in both literal and figurative ways.

Not for this meaning.

To say The heart is important, the normal Arabic sentence is:

• القلب مهم

If you say مهم القلب, that does not work as a normal equivalent of this sentence.

Arabic usually needs the subject first in this kind of simple nominal sentence, especially at the beginner level:

• القلب مهم

So it is best to learn this pattern as:

• [noun] + [adjective]
• القلب مهم

A common way is:

• القلب ليس مهمًا = The heart is not important

Here:

• ليس = is not
• مهمًا is the predicate after ليس

If you are studying only very basic nominal sentences, first learn:

• القلب مهم = The heart is important

Then later you can learn negation with ليس:

• القلب ليس مهمًا = The heart is not important