ציר המשקפיים הוא חלק ממסגרת המשקפיים.

Here ציר means hinge.

More generally, ציר can also mean things like axis, pivot, or axle, depending on context. But with glasses, the natural meaning is hinge — the small joint that lets the arm of the glasses fold.

Yes — משקפיים is a plural/dual-form noun.

The ending ־יים often marks the dual in Hebrew, especially for things that come in pairs. A native English speaker can think of it a bit like words such as glasses, pants, or scissors, which are normally talked about in the plural in English too.

So משקפיים is the normal word for glasses or eyeglasses, and it usually appears in this form rather than as a regular singular noun.

Hebrew often prefers a construct chain for X of Y.

So:

• ציר המשקפיים = the hinge of the glasses
• מסגרת המשקפיים = the frame of the glasses

This is more compact and very natural Hebrew.

You can sometimes use של in Hebrew, but in a sentence like this, the construct form sounds more standard and more elegant.

That is because of the construct chain.

In Hebrew, when two nouns are joined in a construct relationship:

• the first noun usually does not take ה־
• the second noun can take ה־
• the whole phrase becomes definite because of the second noun

So:

• ציר המשקפיים means the hinge of the glasses
• מסגרת המשקפיים means the frame of the glasses

Even though ציר and מסגרת do not have ה־, the whole noun phrases are definite.

Sort of, yes.

In present-tense Hebrew, there is usually no normal verb "to be". But when Hebrew connects two noun phrases, it often uses a pronoun like הוא as a copular pronoun.

So in this sentence, הוא functions like is:

• ציר המשקפיים הוא חלק...
• The hinge of the glasses is part...

It is not exactly the same as an English verb, but in practice that is how you should understand it here.

Because הוא agrees with the subject, not with the noun after it.

The subject is:

• ציר המשקפיים = the hinge of the glasses

The main noun there is ציר, and ציר is masculine singular, so Hebrew uses הוא.

Even though מסגרת is feminine, it is part of the predicate, not the subject, so it does not control the choice of הוא here.

Because the sentence means is part of..., not is the part of...

In English, we also usually say:

• The hinge is part of the frame

not

• The hinge is the part of the frame

So Hebrew uses חלק without ה־.

If you said החלק, that would usually sound much more specific, as if you meant the particular part, which is not the idea here.

Because this is the preposition מ־ meaning from / of attached to the noun מסגרת.

In the expression חלק מ־..., the מ־ means of in the sense of part of.

So:

• מ־
  • מסגרת
• becomes ממסגרת

There are two מ letters simply because:

• the first מ is the preposition
• the second מ is the first letter of מסגרת

So חלק ממסגרת המשקפיים means part of the frame of the glasses.

Because the sentence is being fully explicit:

• ציר המשקפיים = the hinge of the glasses
• מסגרת המשקפיים = the frame of the glasses

Hebrew often repeats the noun like this when it wants to be clear and precise.

If the context is already obvious, you could also say:

• ציר המשקפיים הוא חלק מהמסגרת

That would mean The hinge of the glasses is part of the frame, where the frame is understood to be the frame of the glasses.

So the repetition is not wrong or strange — it is just more explicit.

A simple pronunciation guide is:

tsir ha-mishka-FA-yim hu KHE-lek mi-mis-GE-ret ha-mishka-FA-yim

A few helpful notes:

• צ sounds like ts in cats
• ח in חלק is a throaty sound, like the ch in German Bach or Scottish loch
• משקפיים is stressed near the end: mishka-FA-yim
• מסגרת is stressed on GE: mis-GE-ret

Yes, in principle you could make phrases with של, but it would usually sound less natural here.

For example, a learner might try something like:

• הציר של המשקפיים
• המסגרת של המשקפיים

These are understandable, but the construct versions:

• ציר המשקפיים
• מסגרת המשקפיים

are more idiomatic in this kind of descriptive sentence.

So for this sentence, the version you were given is the one you should aim to recognize and imitate.