يتمضمض صديقي بعد فحص الأسنان.

يتمضمض means he rinses his mouth, he swishes water in his mouth, or sometimes he gargles, depending on context.

It does not mean brushes.
For to brush one’s teeth, Arabic would use something like يفرّش أسنانه.

So in this sentence, the idea is that your friend is rinsing his mouth after the dental exam.

The يـ at the beginning marks the verb as third person masculine singular in the imperfect tense.

So يتمضمض already means he rinses / he is rinsing.

Arabic often does not need a separate word for he because the verb itself already shows the person, number, and gender.
If you added هو, it would usually be for emphasis.

Because Arabic very often uses verb-first word order, especially in straightforward statements.

So:

يتمضمض صديقي بعد فحص الأسنان
is a normal Verb–Subject structure.

You could also say:

صديقي يتمضمض بعد فحص الأسنان

That is also correct, but it sounds a bit more like As for my friend, he rinses... with extra focus on my friend.

The base noun is صديق = friend.

When you attach ـي, it means my:

• صديق = friend
• صديقي = my friend

So ـي is a possessive suffix meaning my.

Also, صديقي normally refers to a male friend.
For a female friend, you would say صديقتي.

Because تمضمض already carries the idea of rinsing the mouth.

So Arabic does not need to say his mouth here. It is understood from the verb itself.

That is why the sentence can simply say:

يتمضمض صديقي
= My friend rinses his mouth / is rinsing his mouth

The Arabic imperfect tense can cover several English ideas, depending on context:

• he rinses
• he is rinsing
• sometimes even he will rinse

In this sentence, without extra context, it could mean either:

• My friend rinses his mouth after the dental exam
• My friend is rinsing his mouth after the dental exam

Arabic often leaves that distinction to context.

Literally, it means:

after the examination of the teeth

More natural English would be:

• after the dental examination
• after the teeth examination
• after the dentist checks the teeth

Here:

• بعد = after
• فحص = examination / checking
• الأسنان = the teeth

So the phrase is built from a noun phrase rather than a full clause.

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

In فحص الأسنان:

• فحص = examination
• الأسنان = the teeth

Together, they mean examination of the teeth.

In an iḍāfa, the first noun usually does not take الـ.
So you say:

• فحص الأسنان = examination of the teeth

not:

• الفحص الأسنان

Also, because the second word الأسنان is definite, the whole phrase becomes definite in meaning.

A fully vocalized version would be:

يَتَمَضْمَضُ صَديقي بَعْدَ فَحْصِ الأَسْنانِ

A few useful notes:

• يَتَمَضْمَضُ ends in ـُ because it is an imperfect verb in the normal indicative form.
• صديقي is the subject of the verb, so it is nominative in function, even though the case ending is not clearly heard because of the attached ـي.
• بعدَ is a time expression meaning after.
• فحصِ is genitive after بعد in this structure.
• الأَسْنانِ is also genitive because it is the second term of the iḍāfa.

A learner does not need to memorize all of that immediately, but it helps explain why the phrase is built this way.

Because it comes from a verb with repeated consonants: تمضمض.

Arabic has some verbs that sound repetitive or echo-like, and they often suggest repeated movement, sound, or action.
That fits this meaning well, because rinsing the mouth involves a repeated swishing action.

So even though it may look unusual at first, it is a normal Arabic verb.

The singular is سِنّ = tooth.

Its common plural is أسنان = teeth.

The plural is used because a dental exam usually involves the teeth in general, not just one tooth. So فحص الأسنان naturally means examining the teeth or a dental examination.

You would make both the subject and the verb feminine:

تتمضمض صديقتي بعد فحص الأسنان.

That means:

My female friend rinses her mouth after the dental examination.

Changes:

• يتمضمض → تتمضمض
• صديقي → صديقتي

So Arabic shows the gender both in the noun and in the verb.