رخصة سير المركبة مهمة.

Because Arabic usually does not use a present-tense verb meaning “is” in simple sentences like this.

So:

• رخصة سير المركبة مهمة literally = The vehicle’s driving/registration license important
• natural English = The vehicle license is important

This is called a nominal sentence in Arabic. In the present tense, Arabic normally leaves out is/are.

If you wanted the past tense, then Arabic would use a verb, for example:

• كانت رخصة سير المركبة مهمة = The vehicle license was important

رخصة means license, permit, or authorization.

It is a feminine noun, which matters because the adjective later in the sentence must match it:

• رخصة = a license
• مهمة = important (feminine)

So the sentence is built around رخصة as the main noun.

سير literally relates to movement, travel, driving, or circulation/operation depending on context.

In the phrase رخصة سير المركبة, it helps form an official expression meaning something like:

• vehicle license
• vehicle registration
• license for the vehicle to be driven/used on the road

So although سير by itself can mean movement or travel, here it is part of a set phrase used in administrative/legal language.

Yes. This is an iḍāfa (genitive/possessive construction), in fact a chain of iḍāfas.

Break it down like this:

• رخصة = license
• سير = movement/driving/operation
• المركبة = the vehicle

Together:

• رخصة سير = license of driving/operation
• رخصة سير المركبة = license of the vehicle’s operation / the vehicle license

A useful way to understand it is:

• رخصة is the main noun
• سير explains what kind of license
• المركبة tells whose driving/operation is meant

So the whole chain acts like one noun phrase.

Because it means the vehicle.

In an iḍāfa, the last noun often determines whether the whole phrase is definite or indefinite.

So:

• رخصة سير مركبة = a vehicle’s driving license / a vehicle license
• رخصة سير المركبة = the vehicle’s driving license / the vehicle license

Since المركبة is definite, the whole phrase رخصة سير المركبة is understood as definite.

Also notice that in iḍāfa:

• the first noun usually does not take ال
• definiteness is often carried by the second noun or final noun in the chain

Because the thing being described, رخصة, is a feminine singular noun.

In Arabic, adjectives agree with the noun they describe in:

• gender
• number
• definiteness

Here:

• رخصة = feminine singular
• so the adjective must be مهمة = important (feminine singular)

Compare:

• الكتاب مهم = The book is important
• الرخصة مهمة = The license is important

Even though رخصة سير المركبة is a longer phrase, the head noun is still رخصة, so the adjective matches that.

Grammatically, it is the predicate of the sentence, but it is the same form as an adjective.

In this sentence:

• رخصة سير المركبة = the subject
• مهمة = the predicate, meaning important

So the sentence structure is:

• [subject] + [predicate]
• رخصة سير المركبة + مهمة

Because the predicate is an adjective-like word, English learners often think of it as an adjective, and that is fine for understanding. But in sentence structure, it is functioning as the predicate: is important.

In fully vocalized formal Arabic, it would be:

رُخْصَةُ سَيْرِ الْمَرْكَبَةِ مُهِمَّةٌ

Here is why:

• رخصةُ: nominative, because it is the subject of the nominal sentence
• سيرِ: genitive, because it is the second term in the iḍāfa with رخصة
• المركبةِ: genitive, because it is also inside the iḍāfa chain
• مهمةٌ: nominative, because it is the predicate

In normal everyday writing, these case endings are usually not written:

• رخصة سير المركبة مهمة

A careful formal pronunciation with case endings is approximately:

rukhṣatu sayri l-markabati muhimmatun

In pause, which is how people often actually say it aloud, it would sound more like:

rukhṣat sayr il-markaba muhimma

A few notes:

• رخصة begins with the strong consonant cluster khṣ
• سير is usually pronounced like sayr
• المركبة has al-, but in connected speech it can sound smoother as l-markaba
• مهمة has a doubled m sound: mu-him-ma

The normal, straightforward order is:

رخصة سير المركبة مهمة

This is the most neutral way to say it.

Arabic can sometimes change word order for style, emphasis, or literary effect, but for a learner, the standard pattern here is best:

• subject first
• predicate second

So stick with:

• رخصة سير المركبة مهمة

rather than trying to rearrange it.

Not exactly. المركبة is more general than car.

It means:

• vehicle

That could include:

• a car
• a truck
• a bus
• another kind of vehicle

If Arabic wanted to say specifically car, a more specific word such as سيارة would usually be used.

So in this sentence, vehicle is the safer translation.

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

So you say:

• رخصةُ سيرِ المركبةِ

not:

• الرخصةُ السيرِ المركبةِ

The definiteness of the whole phrase comes from the last noun being definite:

• المركبة

This is a very important rule in Arabic noun phrases:

• first noun in iḍāfa: usually no ال
• last noun can carry definiteness
• the whole phrase then becomes definite

It is a very common Arabic pattern:

definite noun phrase + indefinite predicate

Here:

• رخصة سير المركبة = a definite noun phrase
• مهمة = an indefinite predicate meaning important

This pattern often corresponds to English:

• The X is Y

For example:

• الكتاب جديد = The book is new
• البيت كبير = The house is big
• رخصة سير المركبة مهمة = The vehicle license is important

So this sentence is a very useful model for basic Arabic grammar.