الطبيب جاد.

It is a nominal sentence: a sentence that starts with a noun, not a verb.

• الطبيب = the subject (mubtadaʾ)
• جاد = the predicate/describing part (khabar)

So the structure is:

• the doctor
  • serious

This is a very common Arabic pattern for saying X is Y in the present tense.

In Arabic, the verb to be is usually not stated in the present tense.

So instead of saying:

• the doctor is serious

Arabic simply says:

• الطبيب جاد

But in other tenses, Arabic does use a verb such as كان:

• كان الطبيب جادًّا = The doctor was serious

So the missing is is completely normal.

A careful full reading is:

• الطَّبِيبُ جَادٌّ
• aṭ-ṭabību jāddun

A few helpful notes:

• In الطبيب, the ل of الـ is assimilated because ط is a sun letter.
• So you pronounce it aṭ-ṭabīb, not al-ṭabīb.
• جاد has a doubled final consonant in full pronunciation: jādd.

In normal pause, many speakers drop the final case endings, so you may hear something closer to:

• aṭ-ṭabīb jādd

Because الطبيب is definite: it means the doctor.

But جاد here is the predicate of the sentence, and predicates like this are usually indefinite in Arabic.

So:

• الطبيب جاد = The doctor is serious

If you made جاد definite too, you would usually get a different meaning:

• الطبيب الجاد = the serious doctor

That is a noun phrase, not a full sentence.

No, not normally.

• الطبيب جاد = a complete sentence: The doctor is serious
• الطبيب الجاد = a noun phrase: the serious doctor

This is an important difference in Arabic:

• predicate adjective in a sentence → usually indefinite
• attributive adjective directly describing a noun → matches the noun in definiteness

So if the adjective is part of the noun phrase, it should also take الـ.

With full case endings, the sentence is:

• الطَّبِيبُ جَادٌّ

Both words are in the nominative case:

• الطبيبُ = nominative subject
• جادٌّ = nominative predicate

In everyday Arabic writing, these short vowels are usually not written, so you usually just see:

• الطبيب جاد

That is completely normal.

Because the word is really جادّ with a shadda on the last letter.

So the full form is:

• جَادٌّ

That means the final د is pronounced as a doubled consonant:

• jāddun, not jādun

In unvowelled everyday writing, the shadda is often omitted, so learners may not see it at first.

Yes, الطبيب is masculine singular.

So the adjective is also masculine singular:

• الطبيب جاد

For a female doctor, you would say:

• الطبيبة جادة

Notice that the adjective changes too. In Arabic, adjectives agree with the noun in:

• gender
• number
• definiteness
• and usually case

So masculine جاد becomes feminine جادة.

It can also carry meanings like:

• serious
• earnest
• sincere
• sometimes diligent, depending on context

So the exact English word may vary a little. In this sentence, it describes the doctor as having a serious or earnest attitude.

The basic and most natural order here is:

• الطبيب جاد

This starts with the subject and is neutral.

Arabic can sometimes change word order for emphasis or style, but for a beginner, this is the pattern to remember:

• definite noun
  • indefinite adjective/predicate

That is one of the most common ways to say X is Y in Modern Standard Arabic.