محور العجلة نظيف.

A careful full reading is:

مَحْوَرُ العَجَلَةِ نَظِيفٌ

A rough transliteration is:

maḥwaru al-ʿajalati naẓīfun

In everyday unvowelled text, learners often just read it approximately as:

maḥwar al-ʿajala naẓīf

Here, محور means axle.

This word can also mean axis, pivot, or even main theme/topic in other contexts, but with العجلة after it, the meaning is clearly axle of the wheel.

This is an iḍāfa construction, often called a genitive construct.

Structure:

• محور = axle
• العجلة = the wheel

Together:

• محور العجلة = the axle of the wheel
• It can also be translated naturally as the wheel’s axle

In Arabic, this construction often replaces English of or ’s.

Because محور is the first noun in an iḍāfa.

In an iḍāfa:

• the first noun normally does not take الـ
• the first noun also does not take tanwīn
• the definiteness of the whole phrase comes from the second noun

So:

• محور عجلة = an axle of a wheel / a wheel axle
• محور العجلة = the axle of the wheel

Even without الـ, محور becomes definite here because the whole iḍāfa is definite.

In Arabic iḍāfa, the order is:

thing possessed/defined + owner/definer

So Arabic says literally:

axle the-wheel

but English says:

the axle of the wheel

So the order is normal for Arabic.

In an iḍāfa construction, the second noun is in the genitive case.

So with full case endings:

• مَحْوَرُ = nominative, because it is the subject of the sentence
• العَجَلَةِ = genitive, because it is the second term of the iḍāfa
• نَظِيفٌ = nominative, because it is the predicate

That is why the fully vocalized sentence is:

مَحْوَرُ العَجَلَةِ نَظِيفٌ

Because in Arabic, in a present-tense nominal sentence, the verb to be is usually omitted.

So:

• محور العجلة نظيف literally looks like the axle of the wheel clean
• but it means The axle of the wheel is clean

This is completely normal Arabic grammar.

If you wanted a past meaning, then Arabic would use a verb, for example:

• كان محور العجلة نظيفًا = The axle of the wheel was clean

Because نظيف describes محور, not العجلة.

• محور is masculine singular
• العجلة is feminine singular

The adjective must agree with the noun it describes, which is محور.

So:

• محور ... نظيف = correct
• محور ... نظيفة = incorrect here

If the sentence were العجلة نظيفة, then نظيفة would be correct, because it would describe العجلة.

Because نظيف is the predicate of the sentence, not an attributive adjective inside the noun phrase.

Arabic distinguishes between:

1. Predicate adjective

   • محور العجلة نظيف
   • The axle of the wheel is clean
   • Here نظيف is usually indefinite

2. Attributive adjective

   • محور العجلة النظيف
   • the clean axle of the wheel
   • Here النظيف is part of the noun phrase, so it is definite and matches the noun phrase

So in your sentence, the indefiniteness of نظيف is exactly what we expect.

It is an adjective used as the predicate of the sentence.

So grammatically, the sentence has:

• محور العجلة = the subject
• نظيف = the predicate adjective

This is very common in Arabic nominal sentences.

Yes. That is a very natural English translation.

Both of these are good:

• The axle of the wheel is clean
• The wheel’s axle is clean

That is because the Arabic iḍāfa محور العجلة can often be translated with either of or ’s.

That would be a different sentence.

• محور العجلة نظيف = The axle of the wheel is clean
• العجلة نظيفة = The wheel is clean

So the original sentence is specifically about the axle, not the whole wheel.