ذراع النظارة جزء من إطار النظارة.

This is a nominal sentence in Arabic. In the present tense, Arabic usually does not use a separate word for is / are.

So:

• ذراع النظارة = the arm of the glasses
• جزء من إطار النظارة = a part of the frame of the glasses

Together, the sentence naturally means The arm of the glasses is part of the frame of the glasses.

If you wanted was, then Arabic would normally use كان.

This is an iḍāfa construction, often called a genitive construction or noun + of + noun structure.

• ذراع = arm
• النظارة = the glasses / the eyeglasses

So ذراع النظارة literally means arm of the glasses.

Important things about iḍāfa:

1. The first noun usually does not take ال.
2. The second noun is often definite if the whole phrase is definite.
3. In full case marking, the second noun is in the genitive.

So:

• ذراعُ النظارةِ = the arm of the glasses

Because the sentence contains two separate iḍāfa phrases:

1. ذراع النظارة = the arm of the glasses
2. إطار النظارة = the frame of the glasses

Arabic often repeats the noun instead of replacing it with a pronoun if that makes the structure clear and natural.

So the sentence is literally:

• The arm of the glasses
• is a part of
• the frame of the glasses

Because here it means a part, not the part.

In Arabic, the predicate (خبر) of a nominal sentence is often indefinite, especially when it describes what something is.

So:

• ذراع النظارة جزء... = The glasses arm is a part...
• ذراع النظارة الجزء... would not work here in the same way

This is similar to English:

• The arm is a part of the frame not
• The arm is the part of the frame

Here من means of in the sense of part of.

So:

• جزء من إطار النظارة = a part of the glasses frame

This is a very common pattern:

• جزء من الكتاب = a part of the book
• جزء من البيت = a part of the house

Also remember: after من, the following noun is in the genitive when fully vowelled.

Grammatically, نظارة is singular in form, but it can refer to a pair of glasses / eyeglasses.

This is similar to how some objects are treated differently across languages. English uses the plural glasses, but Arabic often uses a singular noun for the object as a whole.

So in this sentence:

• النظارة = the glasses / the eyeglasses

You may also encounter نظارات in other contexts, often meaning glasses in a more general or plural sense, but نظارة for a single pair of eyeglasses is very common.

Yes, ذراع literally means arm, but like English arm, it can also be used for the side piece of glasses.

So in this context, ذراع النظارة means the arm or temple of the glasses.

This is a normal extension of meaning:

• human arm
• arm of a chair
• arm of glasses

Context tells you which meaning is intended.

A fully vowelled version would be:

ذِراعُ النَّظَّارَةِ جُزْءٌ مِنْ إِطارِ النَّظَّارَةِ.

This shows the grammar more clearly:

• ذِراعُ = nominative, because it is the subject of the nominal sentence
• النَّظَّارَةِ = genitive, because it is the second term of an iḍāfa
• جُزْءٌ = nominative, because it is the predicate
• إِطارِ = genitive after من
• النَّظَّارَةِ = genitive as the second term of another iḍāfa

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

Because in an iḍāfa construction, definiteness comes from the whole phrase, not necessarily from putting ال on the first noun.

So:

• ذراع نظارة = an arm of glasses / a glasses arm
• ذراع النظارة = the arm of the glasses

The first noun, ذراع, becomes definite because it is attached to the definite second noun النظارة.

This is a key feature of iḍāfa in Arabic.

Yes, but that would mean the two arms of the glasses.

• ذراع النظارة = the arm of the glasses / one arm
• ذراعا النظارة = the two arms of the glasses

So the sentence you have is talking about one arm as a part of the frame. If you wanted to talk about both side pieces together, the dual would be possible.

Yes. إطار النظارة is another iḍāfa phrase:

• إطار = frame
• النظارة = the glasses

So:

• إطار النظارة = the frame of the glasses

This means the sentence contains one iḍāfa inside another larger sentence structure:

• ذراع النظارة = subject
• جزء من إطار النظارة = predicate phrase

This is very common in Arabic technical and descriptive language.

Yes. A very literal translation is:

• The arm of the glasses is a part of the frame of the glasses.

But more natural English might be:

• The arm of the glasses is part of the frame.
• A glasses arm is part of the frame.
• The temple of the glasses is part of the frame.

Arabic often repeats nouns more explicitly than English does, while English may drop repeated information if it is already understood.