يخدر طبيب الأسنان السن بعد فحص الأسنان.

The sentence is:

يخدر — numbs / anesthetizes
طبيب الأسنان — the dentist
السن — the tooth
بعد فحص الأسنان — after examining the teeth / after the examination of the teeth

So the pattern is:

verb + subject + object + time phrase

In other words, Arabic is saying:

Numbs the dentist the tooth after examining the teeth,
which in natural English becomes The dentist numbs the tooth after examining the teeth.

Because in Modern Standard Arabic, a verb-first sentence is very common and often feels neutral or formal.

So:

يخدر طبيب الأسنان السن
is a normal VSO pattern: verb–subject–object

English usually prefers SVO: The dentist numbs the tooth.

Arabic can also use subject-first word order:

طبيب الأسنان يخدر السن بعد فحص الأسنان

That is also correct, but starting with the verb is very common in MSA.

يخدر here means he numbs or he anesthetizes.

More fully vocalized, it is usually:

يُخَدِّرُ

This is the non-past form in Arabic, which can express:

• a present action
• a habitual action
• a general procedure

So in this kind of sentence, it often describes a standard action in a procedure, like:

The dentist numbs the tooth after examining the teeth.

Also, the doubled middle consonant is important: it is pronounced with a shadda on د, even though the sentence as written does not show short vowels or shadda.

Because طبيب الأسنان is an iḍāfa construction, often called a construct phrase.

Literally, it means:

doctor of the teeth

In an iḍāfa:

• the first noun usually does not take ال
• the second noun determines whether the whole phrase is definite or indefinite

So:

• طبيب أسنان = a dentist
• طبيب الأسنان = the dentist

That is why طبيب has no ال, but the whole phrase is still definite.

Both are examples of iḍāfa.

1. طبيب الأسنان

   • طبيب = doctor
   • الأسنان = the teeth
   • together: the dentist / literally doctor of the teeth

2. فحص الأسنان

   • فحص = examination / examining
   • الأسنان = the teeth
   • together: the examination of the teeth / examining the teeth

A key rule of iḍāfa is that the second noun is in the genitive relation. If full case endings were written, you would see that in forms like:

• طبيبُ الأسنانِ
• فحصِ الأسنانِ

Because Arabic often uses a verbal noun (called a maṣdar) after words like بعد.

Here:

• بعد = after
• فحص = examination / examining

So:

بعد فحص الأسنان
literally means after the examination of the teeth

In natural English, that is often best translated as:

after examining the teeth

Arabic could also use a full clause, for example:

بعد أن يفحص الأسنان
= after he examines the teeth

Both are possible. The maṣdar version is compact and very common in formal Arabic.

Because the sentence is talking about:

• one specific tooth that will be numbed: السن
• the teeth in general being examined first: الأسنان

This is perfectly natural in a dental context:

• first the dentist examines the teeth
• then he numbs the specific tooth that needs treatment

Also, أسنان is the plural of سن. It is a broken plural, so the singular and plural do not just differ by a simple ending.

Because the tooth is understood as a specific tooth in the situation.

So السن means the tooth, not just a tooth.

In context, this makes sense: at the dentist, there is usually a known tooth being treated. Arabic often uses the definite article when the referent is specific or already understood from the situation.

If you said سنًّا, that would mean a tooth, which is more indefinite and less natural in this specific procedural context.

Yes, that is also correct.

Compare:

• يخدر طبيب الأسنان السن بعد فحص الأسنان
  more neutral, standard verb-first style

• طبيب الأسنان يخدر السن بعد فحص الأسنان
  subject-first, which can sound a bit more like The dentist, he numbs the tooth...

In many contexts, the difference is small. Both are grammatical. The original sentence simply uses a very common MSA word order.

A fully vocalized version is:

يُخَدِّرُ طَبِيبُ الْأَسْنَانِ السِّنَّ بَعْدَ فَحْصِ الْأَسْنَانِ

A helpful pronunciation guide is:

yukhaddiru ṭabību al-asnāni as-sinna baʿda faḥṣi al-asnāni

A few useful things to notice:

• يُخَدِّرُ has a doubled د sound
• السِّنَّ is pronounced as-sinna, not al-sinna, because س is a sun letter, so the ل of ال assimilates
• الأسنان keeps the l sound in careful pronunciation: al-asnān

If you are also learning case endings, this sentence shows them clearly:

• طبيبُ = nominative, because it is the subject
• السنَّ = accusative, because it is the direct object
• فحصِ = genitive in the iḍāfa after بعد
• الأسنانِ = genitive as the second noun of the iḍāfa