إعلان البيع واضح، إذن يشتري المشتري الدراجة المستعملة.

It is an iḍāfa construction, often called a genitive construction.

• إعلان = announcement / advertisement
• البيع = the sale / selling

So إعلان البيع literally means the advertisement of the sale or the sale advertisement.

A key point about iḍāfa:

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

Because البيع is definite, إعلان البيع is understood as definite too.

Because Arabic normally omits the present-tense copula in simple sentences like this.

So:

• إعلان البيع واضح
• literally: The sale advertisement clear
• natural English: The sale advertisement is clear

This is called a nominal sentence in Arabic.

If you wanted was or will be, then Arabic would usually use a verb such as كان or سيكون.

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

So the predicate comes in the matching masculine singular form:

• إعلان ... واضح
• not واضحة

It is not agreeing with الدراجة, because واضح belongs to the first clause: إعلان البيع واضح.

Here إذن means something like so, then, or therefore.

It links the first idea to the result:

• إعلان البيع واضح
• إذن ...
• The sale advertisement is clear, so/therefore ...

In careful traditional grammar, إذن can affect the mood of the following imperfect verb under certain conditions. In everyday modern written Arabic, learners will often just understand it mainly as a connector meaning so / then / therefore.

Because Arabic often prefers verb–subject–object order.

So:

• يشتري المشتري الدراجة
• literally: buys the buyer the bicycle

This is normal Arabic word order.

You can also say:

• المشتري يشتري الدراجة

That is also grammatical, but it gives a slightly different feel, often with more focus on the buyer.

Because it has the definite article الـ.

• المشتري = the buyer
• مشتري = a buyer (in basic, unvocalized writing)

So in this sentence, the noun refers to a specific buyer, not just any buyer.

Also, المشتري is an active-participle type noun from the same root as يشتري, so the pair is literally something like the buyer buys.

Because Arabic adjectives normally come after the noun they describe.

So:

• الدراجة المستعملة
• literally: the bicycle the-used
• natural English: the used bicycle

This is the normal order in Arabic:

• noun first
• adjective second

Because it must agree with الدراجة, and دراجة is a feminine singular noun.

Arabic adjectives agree with the noun in:

• gender
• number
• definiteness
• and, in fully vocalized Arabic, case

So:

• الدراجة = feminine singular definite
• therefore المستعملة must also be feminine singular definite

That is why you get الدراجة المستعملة, not الدراجة المستعمل.

The Arabic imperfect verb form, such as يشتري, can express:

• present
• habitual action
• near future
• or a general result, depending on context

Here, after إذن, it can be understood as a consequence: so the buyer buys / will buy the used bicycle.

If you wanted to make the future meaning clearer, Arabic could also say:

• إذن سيشتري المشتري الدراجة المستعملة

That would more clearly mean So the buyer will buy the used bicycle.

It is grammatically fine, and Arabic does allow this kind of repetition. But stylistically, some speakers or writers might feel it sounds a little repetitive, because both words come from the same root related to buying.

So the sentence is understandable, but a writer might choose a smoother alternative such as:

• إذن يشتري الزبون الدراجة المستعملة
• لذلك يشتري المشتري الدراجة المستعملة
• إذن سيشتري المشتري الدراجة المستعملة

So the original is not wrong, but it may sound slightly mechanical or textbook-like.

In a fully vocalized form, you could write it like this:

إعلانُ البيعِ واضحٌ، إذن يشتري المشتريُ الدراجةَ المستعملةَ.

Here is the basic breakdown:

• إعلانُ: nominative, because it is the subject of the nominal sentence
• البيعِ: genitive, because it is the second term of the iḍāfa
• واضحٌ: nominative, because it is the predicate
• المشتريُ: nominative, because it is the subject of يشتري
• الدراجةَ: accusative, because it is the direct object
• المستعملةَ: accusative, because it is an adjective describing الدراجة

In normal everyday Arabic writing, these short vowels are usually not written, but the grammatical relationships are still there.