أما مسند الدراجة فهو ليس جديدا.

أما is a topic-setting word. In sentences like this, it usually means as for..., regarding..., or when it comes to....

So:

• أما مسند الدراجة... = As for the bicycle seat/backrest...

This construction often introduces one topic and then gives a comment about it.

After أما, Arabic very often uses فـ before the comment that follows. This is a very common pattern:

• أما ... فـ ...

So in this sentence:

• أما مسند الدراجة فهو ليس جديدا

the فـ links the topic to what is being said about it.

You can think of it as part of the fixed pattern rather than a word that always needs a separate translation.

هو is a resumptive pronoun, referring back to مسند الدراجة.

So the structure is roughly:

• As for the bicycle seat, it is not new.

Arabic often likes to repeat the reference with a pronoun after a fronted topic. This makes the sentence feel clear and natural.

So:

• أما مسند الدراجة فهو ليس جديدا = very natural
• أما مسند الدراجة فليس جديدا = possible in some contexts, but فهو is smoother and more explicit

It is an iḍāfa construction, often called a genitive construction or noun annexation.

It consists of:

• مسند = support / backrest / seat-like support
• الدراجة = the bicycle

Together:

• مسند الدراجة = the bicycle's seat/support/backrest

A key rule of iḍāfa:

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

So because الدراجة is definite, مسند الدراجة is also definite as a whole.

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

So Arabic says:

• مسند الدراجة

not:

• المسند الدراجة

Even though مسند does not have الـ, the whole phrase is definite because the second noun is definite:

• الدراجة = the bicycle
• therefore مسند الدراجة = the bicycle seat/support

In fully vocalized Arabic, this would be:

• أمّا مسندُ الدراجةِ فهو ليس جديدًا

The endings are:

• مسندُ → nominative
• الدراجةِ → genitive

Why?

• مسندُ is the topic/subject-like noun after أما
• الدراجةِ is the second term of the iḍāfa, and that second term is genitive

So the phrase is:

• مسندُ الدراجةِ

ليس is commonly used to negate a nominal sentence, especially when English would use is not / are not.

The positive idea would be:

• هو جديدٌ = it is new

Negating that:

• هو ليس جديدًا = it is not new

That is why ليس fits naturally here.

By contrast, لا is used in other kinds of negation and would not be the normal choice for this sentence.

Because جديدًا here is not an adjective directly attached to مسند الدراجة inside the noun phrase. It is the predicate of the sentence.

Compare:

1. مسند الدراجة الجديد
   = the new bicycle seat
   Here الجديد is an adjective modifying the noun.

2. مسند الدراجة ليس جديدًا
   = the bicycle seat is not new
   Here جديدًا is the predicate.

In Arabic, predicates are usually indefinite, even when the subject is definite.

So:

• الكتاب جديدٌ = the book is new
• not normally الكتاب الجديد if you mean the book is new

Because ليس behaves like the verb كان in grammar:

• its subject is nominative
• its predicate is accusative

So in:

• فهو ليس جديدًا

the analysis is:

• هو = the subject of ليس
• جديدًا = the predicate of ليس, therefore accusative

That is why you see:

• جديدًا with the accusative ending

In unvocalized text, it is often written simply as جديدا, but the full form is جديدًا.

Yes. A more neutral version would be:

• مسند الدراجة ليس جديدًا

This simply means:

• The bicycle seat/backrest is not new

Using أما changes the feel of the sentence. It gives a topic-comment structure:

• As for the bicycle seat, it is not new.

So أما is useful when contrasting topics or when organizing information in a more formal or structured way.