اللويحة السنية قرب اللثة.

Because Arabic usually omits the present-tense copula (is / are / am) in simple statements.

So اللويحة السنية قرب اللثة is a normal present-time nominal sentence:

• اللويحة السنية = the subject
• قرب اللثة = the predicate/resulting description

If you wanted past or future, Arabic would normally use a verb, for example:

• كانت اللويحة السنية قرب اللثة = the dental plaque was near the gum

Yes. It is a noun + adjective combination:

• اللويحة = plaque
• السنية = dental

So the structure is:

• noun: اللويحة
• adjective modifying it: السنية

This is the ordinary Arabic way to say something like dental plaque.

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

So if the noun is definite:

• اللويحة = the plaque

then the adjective must also be definite:

• السنية = the dental

Even though English says dental plaque rather than the dental plaque, Arabic marks both words as definite:

• اللويحة السنية

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

A quick way to tell:

• In a noun + adjective phrase, the adjective agrees with the noun and can take الـ
• In an iḍāfa, the second word is a noun, not an adjective

Here السنية is an adjective meaning dental, so this is adjective agreement, not possession or of-construction.

Compare:

• اللويحة السنية = the dental plaque
• لويحة الأسنان = plaque of the teeth / tooth plaque

السنية here means dental.

It comes from سِنّ meaning tooth, and uses the common nisba ending ـيّ to form an adjective:

• سِنّ = tooth
• سِنّيّ = dental, relating to teeth
• سِنّيّة = feminine singular form of that adjective

So السنية agrees with اللويحة, which is feminine singular.

Because اللويحة is feminine singular, and Arabic adjectives must agree with the nouns they describe.

Agreement here includes:

• gender: feminine
• number: singular
• definiteness: definite

That is why you get:

• اللويحة السنية

and not a masculine form such as السني.

For a learner, the easiest way to understand قرب here is as a word meaning near.

More technically, it is often analyzed as a noun/adverbial expression (ظرف) meaning nearness, and it is followed by a noun in the genitive:

• قرب اللثة = near the gum

So in this sentence, قرب اللثة functions like the predicate: it tells you where the plaque is.

Both are possible, but they are built differently.

1. قرب اللثة

   • uses قرب = near
   • short and compact
   • common in labels, captions, and concise medical wording

2. قريبة من اللثة

   • uses the adjective قريبة = near
   • followed by من
   • more like a full descriptive sentence

So:

• اللويحة السنية قرب اللثة
• اللويحة السنية قريبة من اللثة

Both can express a similar idea, but the grammar is different.

Arabic often uses a singular body-part noun to refer to the tissue or area in a general way.

So اللثة can mean:

• the gum
• the gum tissue
• the gum area

This is natural in medical or anatomical language. English may prefer gums in some contexts, but Arabic does not have to match that number choice exactly.

A fully vocalized version would be:

اللُّوَيْحَةُ السِّنِّيَةُ قُرْبَ اللِّثَةِ

The grammar behind the endings is:

• اللُّوَيْحَةُ: subject, so nominative
• السِّنِّيَةُ: adjective matching the subject, also nominative
• قُرْبَ: adverbial expression, accusative
• اللِّثَةِ: after قرب, in the genitive

In normal Arabic writing, these short vowels are usually omitted.

A careful pronunciation is approximately:

al-luwayḥatu s-sinniyyatu qurbal-lithati

A few useful notes:

• In اللويحة, the word begins with ل, so you hear a doubled l sound.
• In السنية, the س is a sun letter, so الـ is pronounced as as-, giving as-sinniyya.
• In اللثة, again you hear a doubled l sound because the word begins with ل.

In pause, speakers often drop the final short vowels, so you may hear something closer to:

al-luwayḥa as-sinniyya qurb al-litha

It is a nominal sentence:

• اللويحة السنية = the topic/subject
• قرب اللثة = the predicate describing location

So the pattern is:

[noun phrase] + [location expression]

This is a very common structure in Arabic for saying where something is.