صمام الإطار على الأنبوب الداخلي.

Because Arabic often leaves out the present-tense verb to be.

So صمام الإطار على الأنبوب الداخلي is a nominal sentence:

• صمام الإطار = the subject
• على الأنبوب الداخلي = the predicate, a prepositional phrase

So the sense is simply The tire valve is on the inner tube, even though Arabic does not say is here.

If you wanted the past tense, you would usually add a verb such as كان:

• كان صمام الإطار على الأنبوب الداخلي = The tire valve was on the inner tube

Because صمام الإطار is an iḍāfa construction, often called a genitive construction or possessive construction.

In an iḍāfa:

• the first noun usually does not take الـ
• the whole phrase can still be definite if the second noun is definite

So:

• صمام = valve
• الإطار = the tire
• صمام الإطار = the tire’s valve or more naturally the tire valve

Even without الـ on صمام, the whole phrase is definite because الإطار is definite.

Yes. That is the basic literal structure.

Arabic often expresses relationships between nouns with iḍāfa instead of using of or making a noun-noun compound the way English does.

So:

• صمام الإطار literally = valve of the tire
• natural English = the tire valve

This is very common in Arabic. Compare:

• باب البيت = the door of the house / the house door
• كتاب الطالب = the student’s book

A fully vocalized version would be:

صَمَّامُ الإِطَارِ عَلَى الأُنْبُوبِ الدَّاخِلِيِّ

Here is why:

• صَمَّامُ is nominative because it is the subject of the nominal sentence
• الإِطَارِ is genitive because it is the second term of the iḍāfa
• الأُنْبُوبِ is genitive because it comes after the preposition على
• الدَّاخِلِيِّ is genitive because it is an adjective describing الأنبوب, so it matches it in case

In normal Arabic writing, these short vowels are usually omitted, so learners have to infer them from grammar.

Because الداخلي is an adjective modifying الأنبوب.

In Arabic, adjectives normally:

• come after the noun
• agree with the noun in definiteness, gender, number, and case

So:

• الأنبوب = the tube
• الداخلي = inner / internal
• الأنبوب الداخلي = the inner tube

Since الأنبوب is definite, the adjective must also be definite:

• الأنبوب الداخلي = the inner tube
• أنبوب داخلي = an inner tube

A good clue is the matching definiteness.

In الأنبوب الداخلي:

• الأنبوب has الـ
• الداخلي also has الـ

That is typical for a noun + adjective phrase.

In an iḍāfa, the first noun normally does not take الـ. For example:

• أنبوب الهواء = the air tube / tube of air

So الأنبوب الداخلي is best understood as:

• noun: الأنبوب
• adjective: الداخلي

That gives the inner tube.

على usually means on, upon, or sometimes attached to / on the surface of.

Here it describes the valve as being on or mounted on the inner tube. That is a normal use of على for physical attachment or position.

A learner might expect في because English sometimes says something is in something, but على is appropriate when the idea is location on a surface or attached to an object.

So:

• على الأنبوب الداخلي = on the inner tube

Because د is a sun letter.

When الـ comes before a sun letter, the l sound is not pronounced, and the following consonant is doubled in pronunciation.

So:

• الداخلي is pronounced ad-dākhilī, not al-dākhilī

By contrast:

• الإطار keeps the l sound, because its first consonant is not a sun letter in this pattern of pronunciation
• الأنبوب is also pronounced with the l sound

A careful pronunciation of the whole sentence is roughly:

ṣammāmu l-iṭāri ʿalā l-unbūbi d-dākhiliyyi

In pause, many learners will hear or say something simpler, like:

ṣammām al-iṭār ʿalā al-unbūb ad-dākhilī

In this sentence, الإطار means the tire.

But this word can have other meanings in other contexts, such as:

• frame
• framework
• border
• sometimes tire

So learners may be confused when they see it for the first time. Context decides the meaning.

Here, because the sentence is about a valve and an inner tube, الإطار is understood as tire.