هذه المهارة في المستوى المتوسط.

Because المهارة (skill) is a feminine noun, and the demonstrative must match it.

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

So:

• هذه المهارة = this skill

A very common clue that a noun is feminine is the ending ـة (taa marbuuTa), which you can see in مهارة.

In Arabic, many nouns ending in ـة are grammatically feminine, and مهارة is one of them.

So even though English nouns usually do not have grammatical gender, Arabic nouns do. That means other words in the sentence may need to agree with المهارة as feminine.

Here, that is why you get:

• هذه المهارة not
• هذا المهارة

In Arabic, the present tense of to be is usually not stated in simple sentences like this.

So:

• هذه المهارة في المستوى المتوسط literally looks like:
• This skill in the intermediate level

But it means:

• This skill is at the intermediate level

This kind of sentence is called a nominal sentence. Arabic often leaves out is / am / are in the present tense.

In Arabic, when a noun comes after a demonstrative like هذا or هذه, the noun is usually definite, so it takes ال.

That is why Arabic says:

• هذه المهارة

Literally, this looks like this the-skill, but it is the normal Arabic way to say this skill.

So the ال is not extra or wrong; it is required by standard Arabic structure.

في is the preposition in or at, depending on context.

Here:

• في المستوى المتوسط = at the intermediate level / in the intermediate level

In natural English, at the intermediate level is the better translation.

So في introduces the phrase that tells you where the skill belongs in terms of level.

المستوى has ال because the phrase means the intermediate level, not just an intermediate level or some level.

Compare:

• في المستوى المتوسط = at the intermediate level
• في مستوى متوسط = at an intermediate level

Both can be grammatical, but they do not mean exactly the same thing. The version with ال is more specific and definite.

Because المتوسط is an adjective describing المستوى, and in Arabic adjectives must match the nouns they describe in definiteness.

So if the noun is definite:

• المستوى = the level

then the adjective must also be definite:

• المتوسط = the intermediate

Together:

• المستوى المتوسط = the intermediate level

This is a very important Arabic rule:

• definite noun + definite adjective
• indefinite noun + indefinite adjective

So:

• مستوى متوسط = an intermediate level
• المستوى المتوسط = the intermediate level

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

So:

• المستوى المتوسط literally = the level the-intermediate

But in English we reverse that order:

• the intermediate level

This is completely normal in Arabic. You will see it all the time:

• كتاب جديد = a new book
• الكتاب الجديد = the new book

Yes. After a preposition like في, the following noun is in the genitive case.

So grammatically, المستوى is majruur because it comes after في.

In fully vocalized Arabic, you would see that in the ending. The full form would be something like:

• هذه المهارةُ في المستوى المتوسِّطِ

In normal unvocalized writing, you usually do not see the case ending, but the grammatical rule is still there.

Here it is an adjective, meaning intermediate or middle.

It describes المستوى:

• المستوى المتوسط = the intermediate level

In other contexts, المتوسط can sometimes function more like a noun, depending on the sentence, but here it is clearly describing the noun before it.

A careful pronunciation would be:

hādhihi al-mahārah fī al-mustawā al-mutawassiṭ

If you include full grammatical endings in very formal reading, it would be closer to:

hādhihi al-mahāratu fī al-mustawā al-mutawassiṭi

A few pronunciation notes:

• هذه = hādhihi
• مهارة = mahārah
• مستوى = mustawā
• متوسط = mutawassiṭ

In normal speech, final case endings are often not emphasized.

Yes, that is possible, but the meaning changes a little.

• هذه المهارة في المستوى المتوسط = This skill is at the intermediate level
• هذه مهارة في مستوى متوسط = This is a skill at an intermediate level or This is a skill in an intermediate level

The original sentence is more specific and sounds like you are talking about a particular skill and placing it in a particular level category.

So the version with ال feels more definite and more natural for a labeled level or classification system.