السن حساس للبرودة.

It is written السن, but because س is a sun letter, the ل of الـ is not pronounced. So you say as-sinn, not al-sinn.

The س is pronounced doubled, and the word سِنّ itself has a doubled ن as well.

It can mean either tooth or age; the spelling is the same in normal unvowelled Arabic.

You tell from context:

• السن حساس للبرودة clearly means the tooth
• with other contexts, it could mean age

So this is one of those cases where context does the work.

Because Arabic usually leaves out the present-tense verb to be.

So:

• السن حساس للبرودة literally looks like the tooth sensitive to cold
• but it means the tooth is sensitive to cold

This is a very common Arabic pattern called a nominal sentence.

Because حساس here is the predicate of the sentence, not just an adjective inside the noun phrase.

• السن حساس للبرودة = The tooth is sensitive to cold
• السن الحساس للبرودة = the tooth that is sensitive to cold / the cold-sensitive tooth

In other words:

• حساس without الـ works as is sensitive
• الحساس with الـ would be a regular definite adjective modifying السن

Because it agrees with السن, which is singular and masculine.

Arabic adjectives and predicate adjectives agree with the noun in:

• gender
• number

So here:

• السن = masculine singular
• حساس = masculine singular

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

Here لـ gives the sense of to in English sensitive to.

So:

• حساس للبرودة = sensitive to cold

Also, لـ + الـ combine in writing:

• لِ + البرودة becomes للبرودة

So the spelling with two ل letters is completely normal.

Arabic often uses a noun after a preposition in expressions like this.

So instead of something like sensitive to cold using an adjective, Arabic commonly says:

• حساس للبرودة
• literally: sensitive to coldness

That is a natural Arabic structure. A similar alternative you may also hear is حساس للبرد.

With full case endings, it is:

السِّنُّ حَسّاسٌ لِلبُرودَةِ

Breakdown:

• السِّنُّ = nominative
• حَسّاسٌ = nominative
• لِلبُرودَةِ = genitive after the preposition لِ

In normal pause, many speakers do not pronounce those final case endings clearly, especially outside very formal speech.

Yes. Arabic often uses the definite singular to talk about something in a general way.

So السن حساس للبرودة can mean:

• a statement about the tooth in a specific situation, or
• a more general statement about a tooth / teeth in context

If you explicitly wanted teeth, you could use الأسنان instead, but the singular definite form is very normal in general statements.

Yes: it is stronger than just a random description in the moment. It usually means sensitive as a quality or tendency.

So حساس للبرودة suggests that the tooth reacts easily to cold, not just that it happens to be cold right now.