عمود المقعد على هيكل الدراجة.

This is a normal nominal sentence in Arabic. In the present tense, Arabic usually leaves out the verb to be.

So the structure is:

• عمود المقعد = the subject/topic
• على هيكل الدراجة = the predicate, telling you where it is

So Arabic says something like:

• seat post on the bicycle frame

and English supplies is when translating.

If you wanted a past-tense version, then Arabic would normally use a verb such as كان.

عمود المقعد is an iḍāfa construction, often called a construct phrase.

In Arabic, instead of saying:

• the post of the seat

with a separate word for of, Arabic often puts two nouns together:

• عمود = post / column / shaft
• المقعد = the seat

So عمود المقعد literally means post of the seat, which English naturally renders as seat post.

This is a very common Arabic pattern.

Because in an iḍāfa construction, the first noun usually does not take الـ.

So:

• عمود المقعد is correct
• العمود المقعد is not the normal iḍāfa pattern here

The whole phrase becomes definite because the second noun is definite:

• المقعد = the seat

That makes the full phrase definite as well:

• عمود المقعد = the seat post

For the same reason: هيكل الدراجة is also an iḍāfa.

It is built like this:

• هيكل = frame / structure
• الدراجة = the bicycle

Together:

• هيكل الدراجة = the bicycle frame or literally frame of the bicycle

In this pattern, the first noun normally does not take الـ, so الهيكل الدراجة would not be the standard form.

على usually means on, on top of, or more broadly on / upon / attached to, depending on context.

In this sentence, it gives the location or physical attachment of the seat post relative to the bicycle frame.

So the sense is not just abstract location; it can also imply that something is mounted or positioned on something else.

The sentence begins with a complete noun phrase:

• عمود المقعد

That naturally works as the subject/topic.

Then you get a prepositional phrase:

• على هيكل الدراجة

Because it starts with the preposition على, it functions as a location phrase and serves as the predicate.

So the pattern is:

• noun phrase + prepositional phrase

This is a very common Arabic sentence pattern.

In fully vocalized Arabic, it would be:

عَمُودُ الْمَقْعَدِ عَلَى هَيْكَلِ الدَّرَّاجَةِ

Here is why:

• عَمُودُ is nominative because it is the subject of the nominal sentence
• الْمَقْعَدِ is genitive because it is the second noun in the iḍāfa
• هَيْكَلِ is genitive because it comes after the preposition على
• الدَّرَّاجَةِ is genitive because it is the second noun in the iḍāfa هيكل الدراجة

In normal pause or less formal pronunciation, these final case vowels are often not pronounced clearly.

Because د is a sun letter, while م is a moon letter.

So:

• المقعد is pronounced with the l sound of الـ: al-maqʿad
• الدراجة assimilates the l sound: ad-darrāja

That is why the spelling stays the same with الـ, but the pronunciation changes.

No. مقعد is a general word meaning seat, place to sit, or sometimes seat position depending on context.

In this sentence, because the topic is a bicycle, المقعد is understood as the bicycle seat.

So the context tells you which kind of seat is meant.

Yes, عمود literally has meanings like column, pillar, shaft, or post, but Arabic often uses such words for mechanical parts too.

In this context, عمود المقعد is a natural way to refer to the vertical post/shaft connected to the seat.

So the word keeps its core idea of something long, upright, and supporting or connecting another part.