السن حساس للبرودة، كما أن جير الأسنان واضح.

It is pronounced as-sinn.

Two things are happening:

• س is a sun letter, so the ل of الـ is not pronounced separately.
• سِنّ has a doubled ن sound, so you hear -nn at the end.

So:

• السن → as-sinn

This is a very common pronunciation pattern with the definite article before sun letters.

Here it means tooth.

The word سِنّ can refer to either tooth or age, so learners often notice the ambiguity. In real usage, context usually makes it clear.

In this sentence, words like حساس للبرودة and جير الأسنان clearly show that the topic is dental, so السن means the tooth.

Arabic often uses the singular when talking about a body part in a general or specific medical sense.

So السن حساس للبرودة can mean that the tooth or the affected tooth is sensitive to cold.

If you wanted to talk about teeth in general, you could say:

• الأسنان حساسة للبرودة = Teeth are sensitive to cold

So the singular here is natural and does not sound odd in Arabic.

Because السن is grammatically masculine singular, and adjectives must agree with the noun they describe.

So:

• السن = masculine singular
• حساس = masculine singular adjective

If the noun were feminine singular, you would normally use حساسة instead.

Because حساس commonly takes the preposition لـ to express sensitive to.

So:

• حساس للبرودة = sensitive to cold / sensitive to coldness

Also, لِ + الـ becomes لِلـ, which is why you see:

• لِ + البرودة → للبرودة

This is a normal contraction in Arabic spelling.

البرودة literally means coldness or cold.

In medical or everyday contexts, Arabic often uses an abstract noun where English might simply say cold.

So حساس للبرودة is very natural Arabic for:

• sensitive to cold
• sensitive to cold temperatures

Here كما أن works as a connector meaning something like:

• and also
• moreover
• in addition

So it links the second statement to the first.

It does not need to be translated word-for-word as as that or anything like that. In sentences like this, it is best understood as a formal linking expression.

Because this is an iḍāfa construction, often called a possessive or of-phrase structure.

• جير = tartar / calculus
• الأسنان = the teeth

Together:

• جير الأسنان = the tartar of the teeth = dental tartar

In an iḍāfa:

• the first noun normally does not take الـ
• the second noun can be definite

So الجير الأسنان would be incorrect here.

الأسنان is the plural of سِنّ.

So:

• سِنّ = tooth
• أسنان = teeth

This is a broken plural, which means the plural is formed by changing the internal shape of the word rather than just adding an ending.

That is very common in Arabic, and it is something learners gradually get used to.

واضح describes جير الأسنان, and the head noun there is جير, which is grammatically masculine singular.

So the adjective/predicate is also masculine singular:

• جير الأسنان واضح

As for definiteness: predicates in Arabic are often indefinite, even when the subject is definite.

So this is normal:

• جير الأسنان واضح = the dental tartar is evident / noticeable

You do not need الواضح here.

A fully vocalized version is:

• السِّنُّ حَسَّاسٌ لِلبُرودَةِ، كما أنَّ جيرَ الأَسنانِ واضِحٌ.

Basic grammar:

• السِّنُّ = subject of the first nominal sentence, so it is nominative
• حَسَّاسٌ = predicate, also nominative
• لِلبُرودَةِ = prepositional phrase, so البرودة is genitive after لِ

In the second part:

• أنَّ makes its noun accusative
• so جيرَ is accusative
• الأَسنانِ is genitive because it is the second term of the iḍāfa
• واضِحٌ is the predicate of أنَّ, so it is nominative

Even if short vowels are not written in normal text, this is the underlying grammar.