الطبيب يقول إن هذا صحيح.

A careful fully-vowelled reading is:

الطبيبُ يقولُ إنَّ هذا صحيحٌ
aṭ-ṭabību yaqūlu inna hādhā ṣaḥīḥun

A few helpful notes:

• ṭ is an emphatic t
• dh in hādhā sounds like th in this
• ṣ is an emphatic s
• In normal speech, final case vowels are often not clearly pronounced, so you may hear something closer to aṭ-ṭabīb yaqūl inna hādhā ṣaḥīḥ

Because ط is a sun letter.

In Arabic, when الـ comes before a sun letter, the l sound of ال assimilates to the next consonant. So:

• written: الطبيب
• pronounced: aṭ-ṭabīb

The ط is doubled in pronunciation.

This happens in many words, for example:

• الشمس → ash-shams
• الطالب → aṭ-ṭālib

يقول is the present/imperfect tense form of the verb قال, which means to say.

Here, يقول means:

• he says
• or, depending on context, he is saying

It is:

• 3rd person
• masculine
• singular

That matches الطبيب, because doctor here is one male doctor.

Arabic allows both orders:

• الطبيب يقول إن هذا صحيح
• يقول الطبيب إن هذا صحيح

Both are natural.

Very roughly:

• verb first is often the more neutral verbal-sentence order
• subject first can make the subject feel more like the topic: As for the doctor, he says...

For a learner, the important point is that both word orders are possible in Modern Standard Arabic.

Here إنَّ means something like that in English:

• The doctor says that this is correct.

It introduces the statement being reported.

In ordinary Arabic writing, short vowels are usually not written, so you often see إن instead of the fully vowelled إنَّ.

Usually, if you want the meaning says that..., إنَّ is very natural and standard here.

If you leave it out, you would normally restructure the sentence, for example with a pause or quotation style:

• الطبيب يقول: هذا صحيح
• The doctor says: This is correct.

So for learners, يقول إن... is a very good pattern to remember.

Because in Arabic, the present-tense copula is/am/are is usually not expressed.

So:

• هذا صحيح literally looks like
• this correct

but it means:

• this is correct

This is completely normal in Arabic.

If you wanted a past meaning, you would use a verb such as كان:

• كان هذا صحيحًا = This was correct

Excellent question.

Yes, إنَّ normally makes its noun accusative. But هذا is an indeclinable word, so its form does not visibly change.

So in:

• إنَّ هذا صحيحٌ

هذا is grammatically the noun of إنَّ, but it still stays هذا in form.

If the noun were a regular declinable noun, you would see the change more clearly, for example:

• إنَّ الأمرَ صحيحٌ
• Indeed, the matter is correct.

Here الأمرَ shows the accusative ending.

In Arabic, demonstratives do show gender.

• هذا = this for masculine singular
• هذه = this for feminine singular

In your sentence, هذا is masculine. That can be because:

1. it refers to a masculine noun, or
2. it refers more generally to an idea, statement, or situation, where masculine singular is often the default choice

So Arabic forces a gender choice where English does not.

صحيح here is the predicate adjective, meaning correct or true.

So:

• هذا صحيح = this is correct

It is normally indefinite in this kind of sentence, which is why it does not take الـ.

Compare:

• هذا صحيحٌ = this is correct
• هذا الصحيحُ = this is the correct one / this is the correct thing

So adding الـ changes the meaning.

Also, in a fully vowelled reading, صحيحٌ is nominative here, which is why it ends in -un.

The fully vowelled sentence is:

الطبيبُ يقولُ إنَّ هذا صحيحٌ

Word by word:

• الطبيبُ — nominative, because it is the subject
• يقولُ — present tense verb
• إنَّ — particle
• هذا — the noun of إنَّ, in accusative position, but its form does not change
• صحيحٌ — nominative predicate of إنَّ

In normal everyday writing, these short vowels are usually omitted, so learners often see:

الطبيب يقول إن هذا صحيح