هذه المهارة مهمة.

Because مهارة is a feminine singular noun in Arabic, and the demonstrative must match it.

• هذا = this for masculine singular nouns
• هذه = this for feminine singular nouns

So:

• هذا الكتاب = this book
• هذه المهارة = this skill

Even though English uses just this for both, Arabic makes a gender distinction.

In Arabic, a noun after a demonstrative like هذا / هذه is usually still definite, so it normally takes الـ.

So هذه المهارة is the normal MSA way to say this skill.

This is different from English, where we do not say this the skill. In Arabic, that combination is perfectly normal and expected.

Because Arabic usually leaves out the verb to be in the present tense.

So هذه المهارة مهمة literally looks like:

• this skill important

but it naturally means:

• This skill is important

If you want was or will be, Arabic normally uses a verb:

• كانت هذه المهارة مهمة = This skill was important
• ستكون هذه المهارة مهمة = This skill will be important

Because it agrees with المهارة, which is feminine singular.

In Arabic, adjectives usually agree with the noun they describe in:

• gender
• number
• definiteness

Since مهارة is feminine singular, the adjective must also be feminine singular:

• masculine: مهم
• feminine: مهمة

So:

• الكتاب مهم = The book is important
• المهارة مهمة = The skill is important

Because here مهمة is the predicate of the sentence: important in This skill is important.

In Arabic, a predicate adjective in this kind of sentence is usually indefinite:

• المهارة مهمة = The skill is important

But if you want an adjective directly attached to the noun, as in the important skill, then it must match in definiteness:

• المهارة المهمة = the important skill

So the difference is:

• هذه المهارة مهمة = This skill is important
• هذه المهارة المهمة = this important skill

That is a very important distinction in Arabic.

It is a nominal sentence in Arabic.

You can think of it as:

• هذه المهارة = the subject, this skill
• مهمة = the predicate, important

So the pattern is:

• demonstrative + noun = subject
• adjective = predicate

In traditional grammar terms, this is a sentence with a مبتدأ and خبر, though learners do not need those terms right away to understand the structure.

With full case endings, it would be:

هَذِهِ الْمَهَارَةُ مُهِمَّةٌ

The endings show the grammar more clearly:

• الْمَهَارَةُ ends in -u because it is the subject
• مُهِمَّةٌ ends in -un because it is an indefinite predicate

In normal everyday writing, these short vowels are usually omitted, so you normally see:

هذه المهارة مهمة

The final ة is called tāʾ marbūṭa.

In pause, which is how words are usually said at the end of a phrase, it is pronounced like -a:

• مهارة ≈ mahāra
• مهمة ≈ muhimma

If the word is followed by a grammatical ending or connected in certain constructions, the t sound can appear:

• مهارةٌ
• مهارةُ اللغة
• مهمةٌ

So for a beginner, the safest first idea is:

• final ة usually sounds like -a when you stop on the word

A careful pronunciation is:

hādhihi al-mahāratu muhimmatun

In normal pause, many speakers would say it more simply as:

hādhihi al-mahāra muhimma

A few useful pronunciation notes:

• هذه = hādhihi
• مهارة has a long ā
• مهمة has a doubled m sound because of the shadda on م

They mean different things.

• هذه المهارة مهمة = This skill is important
• هذه مهارة مهمة = This is an important skill

Why?

In هذه المهارة مهمة, the phrase هذه المهارة means this skill, and مهمة is the predicate.

In هذه مهارة مهمة, the word مهارة is indefinite, so هذه works more like this is, and مهارة مهمة means an important skill.

So the presence or absence of الـ changes the structure and meaning.