أما واقي الطين فهو فوق محور العجلة.

Here أما means something like as for... or regarding.... It introduces a topic and then comments on it.

So the pattern is:

أما X فهو Y
= As for X, it is Y

In this sentence, أما واقي الطين sets up the mudguard / fender as the topic.

After أما, Arabic very often uses فـ before the comment about the topic. This فـ links the topic to what comes next.

So:

• أما واقي الطين = as for the mudguard
• فهو فوق محور العجلة = it is above the wheel axle

This is a very common structure in formal Arabic.

The هو is a resumptive pronoun: it refers back to واقي الطين and makes the sentence sound complete and natural in this pattern.

So Arabic often says:

أما زيد فهو...
أما الكتاب فهو...
أما واقي الطين فهو...

In English, we often would not repeat the pronoun, but in Arabic this is very common and elegant.

Yes. واقي الطين is an iḍāfa construction, often called a construct phrase.

Literally it is:

• واقي = protector / guard
• الطين = the mud

So literally: mud-protector, meaning mudguard or fender.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun determines whether the whole phrase is definite

Because الطين is definite, واقي الطين as a whole is definite too.

Because in an iḍāfa construction, the first word normally does not take الـ.

So Arabic says:

• واقي الطين = the mudguard
• not الواقي الطين

The definiteness comes from the second noun, الطين.

This is one of the most important patterns in Arabic grammar.

It is another iḍāfa construction:

• محور = axle / axis
• العجلة = the wheel

So محور العجلة means the axle of the wheel or the wheel axle.

Just like with واقي الطين, the first noun has no الـ, and the second noun makes the phrase definite.

In present-tense nominal sentences, Arabic usually does not use a separate word for is / are.

So:

• هو فوق محور العجلة literally looks like it above the wheel axle
• but it means it is above the wheel axle

This is completely normal in Arabic.
If you wanted past tense, then you would use a verb such as كان.

فوق means above / over, while على usually means on / on top of, often with the idea of contact.

So:

• فوق محور العجلة = above the wheel axle
• على محور العجلة would suggest being on the axle itself

Since a mudguard is typically positioned higher than the axle, فوق is the natural choice.

Not here. In this sentence, أما does not mean simple but. It is better understood as as for or regarding.

Sometimes learners see أما and think it is a direct equivalent of but, but in this structure it is really a topic-setting word.

So this sentence is not contrasting two things with plain but; it is introducing one item and saying something about it.

A careful pronunciation is:

ʾammā wāqī aṭ-ṭīni fa-huwa fawqa miḥwari l-ʿajalati

A few notes:

• أما = ʾammā
• الطين is pronounced aṭ-ṭīn because ط is a sun letter, so the l of ال assimilates
• فهو = fa-huwa
• محور = miḥwar
• العجلة = al-ʿajala

A simplified breakdown is:

• أما = topic-introducing particle
• واقي = part of an iḍāfa; it is the topic here
• الطينِ = genitive, because it is the second term of the iḍāfa
• هو = pronoun, the subject of the comment clause
• فوقَ = an adverb-like word of place (above)
• محورِ = genitive after فوق in this construction
• العجلةِ = genitive as the second term of the iḍāfa محور العجلة

So the sentence contains two iḍāfas:

• واقي الطين
• محور العجلة

Yes. Depending on context, واقي الطين can be translated as mudguard or fender.

The exact English word depends on the variety of English and on the object being described:

• British English often prefers mudguard
• American English often prefers fender

So both are reasonable translations.

Yes. The pattern أما ... فهو ... is very useful in Modern Standard Arabic.

You can use it for explanations, comparisons, and organized descriptions:

• أما الباب فهو مفتوح = As for the door, it is open.
• أما السيارة فهي سريعة = As for the car, it is fast.
• أما واقي الطين فهو فوق محور العجلة = As for the mudguard, it is above the wheel axle.

It is especially common in formal writing and careful speech.