اللثة سليمة، كما أن اللويحة السنية على السن.

Because Arabic often uses a nominal sentence in the present tense, with no written or spoken verb for to be.

• اللثة سليمة
• literally: the gum healthy
• natural English: The gum is healthy / The gums are healthy

This is completely normal in Modern Standard Arabic.

اللثة is singular and feminine.

That can feel strange to an English speaker, because English often uses the plural gums when speaking generally. Arabic often uses the singular اللثة to refer to gum tissue or the gum as a body part in a general sense.

So even if the English translation says gums, the Arabic word here is still grammatically singular.

Because adjectives in Arabic must agree with the noun they describe.

Here:

• اللثة = a feminine singular noun
• سليمة = the feminine singular form of سليم (healthy, sound, intact)

So:

• masculine: سليم
• feminine: سليمة

That is why you see سليمة, not سليم.

In this sentence, كما أن works as a connector meaning something like:

• and also
• moreover
• in addition
• as well as

So it introduces another statement after the first one.

Even though كما by itself often means as or just as, in كما أن you should usually understand the whole expression together as a linking phrase.

No. It is not an iḍāfa. It is a noun + adjective phrase.

• اللويحة = the plaque
• السنية = dental

So اللويحة السنية means the dental plaque.

How can you tell it is not iḍāfa?

• In an iḍāfa, the first noun normally does not take ال.
• Here, the first word does have ال: اللويحة
• The second word also has ال and agrees like an adjective: السنية

So this is a standard noun followed by its adjective.

Because in Arabic, an adjective normally matches the noun in definiteness as well as gender and number.

So:

• اللويحة السنية = the dental plaque
• لويحة سنية = a dental plaque

Since the noun is definite, the adjective must also be definite.

على means on or on top of.

So على السن means on the tooth, which fits the idea of plaque being located on the surface of a tooth.

This is a very normal use of على for something attached to or resting on a surface.

Arabic often uses the singular when referring to a thing in a general or representative way, especially in descriptive or medical-style language.

So على السن can mean on the tooth in a generic sense, or on a specific tooth depending on context.

If the writer wanted to talk clearly about teeth in general, they might also say:

• على الأسنان = on the teeth

But the singular in your sentence is not grammatically strange.

Here it is really أنَّ (with emphasis), although short vowels are usually not written in normal Arabic text.

It introduces a new clause. In full vocalization, the second part would be something like:

• كما أنَّ اللويحةَ السنيةَ على السنِّ

Grammatically:

• اللويحةَ is the noun of أنَّ
• على السنِّ is the predicate

In everyday unvowelled writing, you usually do not see these case endings, so a learner may miss that anything is happening grammatically.

Because ordinary Arabic writing usually leaves out short vowels and most case endings.

So although a fully vocalized version would show something like:

• اللِّثةُ سليمةٌ
• كما أنَّ اللُّويحةَ السِّنِّيَّةَ على السِّنِّ

normal text just writes:

• اللثة سليمة، كما أن اللويحة السنية على السن

This is standard in newspapers, books, websites, and most modern printed Arabic.

Yes. س is a sun letter, so the ل of ال is not pronounced separately.

So:

• السن is pronounced roughly as-sinn

The spelling stays the same, but the pronunciation changes.

This is one of the common pronunciation patterns with the Arabic definite article.