بطانة المكابح جيدة، إذن يأخذ المشتري جولة تجريبية.

In Arabic, the present-tense verb to be is usually omitted.

So a structure like:

• بطانة المكابح جيدة

literally looks like brake-lining the-brakes good, but it means The brake lining is good.

This is very normal in Arabic. If you wanted the past, future, or another tense, then Arabic would usually use a form of كان:

• كانت بطانة المكابح جيدة = The brake lining was good

Because بطانة المكابح is an iḍāfa construction, often translated as an of-phrase.

• بطانة = lining / pad
• المكابح = the brakes

Together:

• بطانة المكابح = the lining of the brakes / the brake lining

In an iḍāfa:

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

Since المكابح is definite, the whole phrase is definite.

It is the second part of the iḍāfa and tells you what kind of lining is being talked about.

So:

• بطانة = a lining
• بطانة المكابح = the brake lining / lining of the brakes

Grammatically, the second noun in an iḍāfa is in the genitive case, even though short vowels are usually not written in normal text.

Because it agrees with بطانة, which is a feminine singular noun.

Arabic adjectives normally agree with the noun they describe in:

• gender
• number
• definiteness

So:

• بطانة = feminine singular
• جيدة = feminine singular adjective

That is why you get:

• بطانة المكابح جيدة

not:

• بطانة المكابح جيد

Also, in this kind of sentence, جيدة is a predicate adjective rather than an attributive adjective, so it is normally indefinite.

Because جيدة is not part of the noun phrase بطانة المكابح. It is the predicate of the sentence.

Compare the two ideas:

• بطانة المكابح الجيدة = the good brake lining
  Here الجيدة is an adjective directly attached to the noun phrase.

• بطانة المكابح جيدة = The brake lining is good
  Here جيدة is the predicate: it tells you something about the subject.

That is why it appears without الـ.

Here إذن means something like:

• so
• then
• therefore

It connects the first statement to the conclusion in the second clause:

• بطانة المكابح جيدة، إذن...
• The brake lining is good, so...

So it is functioning as a logical connector.

Yes, and this is very common.

The Arabic imperfect form, such as يأخذ, can express:

• present
• habitual action
• near future
• future from context

So in context, يأخذ المشتري جولة تجريبية can be understood as:

• the buyer takes a test drive
• the buyer will take a test drive

If you wanted to mark the future more explicitly, you could say:

• سيأخذ المشتري جولة تجريبية

Because verb-subject-object order is very common in Arabic.

So this is a normal pattern:

• يأخذ المشتري جولة تجريبية

Literally:

• takes the buyer a test drive

but naturally:

• the buyer takes a test drive

Arabic can also use subject-verb order:

• المشتري يأخذ جولة تجريبية

That version can sound slightly more topic-focused, like as for the buyer, he takes a test drive.

It is a noun here, meaning the buyer.

It comes from the verb اشترى = to buy and has the pattern of an active participle, meaning the one who buys.

So:

• مشتري = buyer
• المشتري = the buyer

Even though it comes from a verb historically, in this sentence it functions simply as a noun: the subject of يأخذ.

Because جولة is a feminine noun, so its adjective must also be feminine:

• جولة = round / trip / tour
• تجريبية = experimental / trial / test

So:

• جولة تجريبية = a test drive or more literally a trial round/trip

This is standard adjective agreement.

Yes. In ordinary Arabic writing, short vowels and most case endings are usually omitted.

So learners often see:

• بطانة المكابح جيدة، إذن يأخذ المشتري جولة تجريبية

But grammatically, words in the sentence have case and mood endings, for example:

• بطانةُ
• المكابحِ
• جيدةٌ
• يأخذُ
• جولةً
• تجريبيةً

These endings are part of the grammar even when they are not written in everyday text.