يعمل صديقي في هذا المجال.

A common fully vowelled version is:

يَعْمَلُ صَدِيقِي فِي هَذَا الْمَجَالِ

This helps show:

• يَعْمَلُ = he works
• صَدِيقِي = my friend
• فِي = in
• هَذَا = this
• الْمَجَالِ = the field / the area / the sector

In everyday printed Arabic, these short vowels are usually omitted.

Arabic often allows the verb to come first, especially in Modern Standard Arabic.

So:

• يعمل صديقي في هذا المجال = My friend works in this field
• literally: Works my friend in this field

This verb-first order is very normal in MSA.

You could also say:

• صديقي يعمل في هذا المجال

That version is also correct and may feel a bit more like My friend works in this field with extra focus on my friend.

يعمل comes from the verb عَمِلَ / يَعْمَلُ, meaning to work or to do/work at.

In this sentence, يعمل means:

• he works
• sometimes, depending on context, he is working

In Arabic, the present tense can cover both simple present and present progressive, depending on context.

The form يعمل is a third-person masculine singular present-tense verb.

The prefix يـ often marks:

• he in the present tense

So:

• يعمل = he works

Because صديقي (my friend) is grammatically masculine here, the verb matches it.

If the subject were feminine, you would usually say:

• تعمل صديقتي = my (female) friend works

The ـي at the end means my.

So:

• صديق = friend
• صديقي = my friend

This is the attached possessive pronoun for my.

A useful pattern:

• كتاب = book
• كتابي = my book
• صديق = friend
• صديقي = my friend

In Arabic, possession is often shown by attaching a pronoun directly to the noun.

So instead of saying something like friend my, Arabic combines them:

• صديقي = my friend

This is very common in Arabic:

• بيتي = my house
• سيارتي = my car
• أستاذي = my teacher

In Modern Standard Arabic, demonstratives like هذا usually come before the noun:

• هذا المجال = this field

Also, when هذا comes before a noun, the noun is usually definite with الـ:

• هذا المجال
• هذه المدينة
• هذا الكتاب

So في هذا المجال literally means:

• in this the-field but naturally: in this field

In some spoken dialects, other word orders can happen, but in MSA هذا المجال is the standard form.

In Arabic, after هذا / هذه / ذلك and similar demonstratives, the noun is normally definite.

So you say:

• هذا الكتاب = this book
• هذه السيارة = this car
• هذا المجال = this field

Using الـ here is just part of the normal grammar pattern in MSA.

المجال often means:

• field
• area
• domain
• sector

In this sentence, it probably means something like:

• this field
• this line of work
• this area

So if someone says:

• يعمل صديقي في هذا المجال it usually means My friend works in this field/profession/sector, not necessarily a physical field.

No separate word for is is needed here, because the main verb is already يعمل = works / is working.

Arabic often does not use a separate present-tense to be the way English does.

For example:

• هو طالب = He is a student
  There is no written is in Arabic.

But in your sentence, the verb is works, so no is is needed anyway.

In full formal Arabic, yes, there are case endings:

• يَعْمَلُ ends with ـُ
• الْمَجَالِ ends with ـِ after the preposition في

So the fully inflected form is:

• يَعْمَلُ صَدِيقِي فِي هَذَا الْمَجَالِ

However, in most modern reading and speaking situations, learners will often hear or use a pause-style pronunciation with fewer clear case endings. In careful formal speech, though, these endings matter.

A useful point:

• After a preposition like في, the noun is in the genitive case, so المجالِ ends in -i.

Because it comes after the preposition في (in).

In Arabic, nouns after prepositions are normally genitive.

So:

• في البيتِ = in the house
• في الكتابِ = in the book
• في هذا المجالِ = in this field

That is why, in full vocalization, you get الْمَجَالِ with kasra at the end.

هذا is usually pronounced hādhā.

A rough guide:

• هَ = ha
• ذَا = dhā

The letter ذ sounds like the th in this for many English speakers.

So:

• هذا ≈ hā-dhā

That makes the phrase:

• في هذا المجال approximately:
• fī hādhā al-majāl

Yes. The Arabic present tense often covers both meanings, and context tells you which one is intended.

So يعمل صديقي في هذا المجال can mean:

• My friend works in this field (general fact)
• My friend is working in this field (current situation)

Without extra context, the first meaning is usually the most natural translation.

Yes. The sentence pattern is very reusable:

• يعمل أبي في هذا المجال = My father works in this field
• تعمل أمي في هذا المجال = My mother works in this field
• يعمل الأستاذ في هذا المجال = The professor works in this field

Just make sure the verb agrees with the subject in gender and number.

A very useful pattern is:

[verb] + [subject] + في + [place/field/area]

For example:

• يعمل خالد في المدرسة = Khalid works at the school
• يدرس الطالب في الجامعة = The student studies at the university
• يسكن أخي في هذه المدينة = My brother lives in this city

From your sentence specifically, you can also learn:

• في هذا المجال = in this field
• صديقي = my friend
• يعمل = he works

So it is a good model sentence for both grammar and vocabulary.