جزء من العمل صعب.

In Arabic, the present tense of to be is usually not stated.
So a sentence like جزء من العمل صعب literally looks like:

• part of the work difficult

But it means:

• Part of the work is difficult

This kind of sentence is very common in Arabic. The linking idea of is is understood automatically.

جزء means part or portion.

A common pronunciation is approximately juzʾ.

A few things to notice:

• The first sound is j
• The middle vowel is u
• The final ء is a hamza, a glottal stop, so the word ends with a little catch: juzʾ

In fully vocalized Arabic, it is often written جُزْء.

من here means of.

So:

• جزء = part
• من العمل = of the work

Together:

• جزء من العمل = part of the work

Arabic often uses من to express the idea of part of something.

العمل means the work, while عمل means work or a work depending on context.

In this sentence, من العمل means of the work, referring to a specific body of work, task, or job.

So:

• من عمل = of work / of a work
• من العمل = of the work

Using ال makes it definite.

This is completely normal in Arabic.

• جزء = a part
• العمل = the work

So the phrase means:

• a part of the work

Arabic does not need both nouns to match in definiteness here. In fact, this combination is very natural:

• جزء من الشيء = a part of the thing

If you said الجزء من العمل, that would usually mean the part of the work, which is a different idea.

Because it describes جزء, and جزء is a masculine singular noun.

So:

• جزء = masculine singular
• صعب = difficult, masculine singular

If the noun were feminine, the adjective would normally be feminine too.

For example:

• مهمة صعبة = a task is difficult / a difficult task

Here, though, جزء is masculine, so صعب is the correct form.

It is an adjective in meaning, but in this sentence it functions as the predicate of the nominal sentence.

The sentence structure is:

• جزء من العمل = the topic / subject-like part
• صعب = what is being said about it

So in English terms, it is like saying:

• Part of the work
  • is difficult

Arabic grammar often calls this kind of sentence a nominal sentence.

Because it is not being used as the difficult part. It is being used predicatively: is difficult.

Compare:

• جزء من العمل صعب = Part of the work is difficult.
• الجزء الصعب من العمل = the difficult part of the work

In the first sentence, صعب tells us something about جزء من العمل.
In the second, الصعب directly modifies الجزء as an attributive adjective.

Yes. Since جزء is indefinite, it can often correspond to:

• a part of the work
• part of the work

Which English version sounds better depends on context. In many cases, Part of the work is difficult is the most natural translation.

In fully inflected Modern Standard Arabic, the sentence would be:

• جُزْءٌ مِنَ العَمَلِ صَعْبٌ

Breakdown:

• جُزْءٌ = nominative, because it is the subject/topic of the nominal sentence
• مِنَ العَمَلِ = after من, العمل is genitive
• صَعْبٌ = nominative, because it is the predicate

In normal writing, these short vowel endings are usually omitted, so you usually just see:

• جزء من العمل صعب

Yes, but the meaning or emphasis may shift.

The basic neutral form is:

• جزء من العمل صعب

You may also see:

• صعبٌ جزءٌ من العمل

but this is less neutral and can sound more literary or marked.

For most learners, the original order is the safest and most natural choice.

You would say:

• الجزء الصعب من العمل

This is different from the original sentence.

Compare:

• جزء من العمل صعب = Part of the work is difficult.
• الجزء الصعب من العمل = The difficult part of the work.

The first is a full sentence.
The second is a noun phrase.

Not exactly in the simple sense of جزء العمل, because the sentence uses من.

A direct iḍāfa is something like:

• جزءُ العملِ = the work’s part / part of the work

But Arabic often prefers جزء من العمل when talking about a part of something in a partitive sense.

So this sentence uses a partitive expression with من, not a plain two-word iḍāfa.

A common way is:

• جزء من العمل ليس صعبًا
  Part of the work is not difficult

Notice that with ليس, the predicate often appears in the accusative in fully inflected Arabic:

• صعبًا

In everyday unvoweled writing, you may still just see:

• جزء من العمل ليس صعبا

You change the noun and then make the adjective agree with it.

For example:

• مهمة من العمل صعبة = A task from the work is difficult
  or more naturally: One task in the work is difficult

Here:

• مهمة is feminine singular
• so the adjective becomes صعبة

For plural:

• أجزاء من العمل صعبة = Parts of the work are difficult

Here:

• أجزاء = parts
• صعبة is the plural non-human agreement form typically used with broken plurals in many contexts

So adjective agreement depends on the noun being described.