Nämä kehykset ovat kevyet, ja uudet linssit tekevät niistä mukavammat.

Nämä is the plural form of tämä.

• tämä = this
• nämä = these

Because kehykset is plural, Finnish uses the plural demonstrative too: Nämä kehykset = these frames.

Yes. In this context, kehykset means frames as in eyeglass frames, and Finnish very naturally uses the plural for one pair.

That is similar to English, where we often say glasses even when we mean one pair. The singular kehys usually means a frame in a more general sense, like a picture frame or one frame as a single object.

So nämä kehykset is the normal way to talk about one pair of eyeglass frames.

Because both subjects are plural.

• Nämä kehykset is plural, so the verb is ovat = are
• uudet linssit is plural, so the verb is tekevät = make

In standard Finnish, verbs agree with plural subjects this way.

A quick comparison:

• kehys on kevyt = the frame is light
• kehykset ovat kevyet = the frames are light

In everyday spoken Finnish, you may often hear singular verb forms with plural subjects, but in standard written Finnish, ovat and tekevät are correct here.

Because adjectives usually agree with the noun they describe in number and case.

Here:

• kevyet describes kehykset
• uudet describes linssit

Since both nouns are plural nominative, the adjectives are also plural nominative.

Their basic forms are:

• kevyt = light
• uusi = new

So:

• kevyt kehys = a light frame
• kevyet kehykset = light frames
• uusi linssi = a new lens
• uudet linssit = new lenses

Niistä is the elative plural form of ne.

• ne = they / those / them
• niistä = from them / out of them / of them

In this sentence, niistä refers back to kehykset.

So even though English just says them, Finnish uses the form niistä because of the grammar of tehdä in this structure.

Because Finnish often uses the pattern:

tehdä jostakin jotakin

This means to make something into something or to make something become something.

So:

• niistä = from them / of them
• mukavammat = more comfortable ones

Very literally, the sentence says something like:

The new lenses make from them more comfortable ones.

That sounds odd in English, so natural English is simply:

The new lenses make them more comfortable.

So niistä is there because Finnish is using a make-from-X-into-Y type of structure.

Mukavammat is the plural comparative form of mukava.

• mukava = comfortable
• mukavampi = more comfortable
• mukavammat = more comfortable (plural)

It is plural because it refers to the plural thing being talked about: the frames.

So the idea is that the frames become more comfortable after getting the new lenses.

Because Finnish can leave out a noun when it is obvious from context.

Here, mukavammat really means something like:

• mukavammat kehykset = more comfortable frames

But since kehykset has already been mentioned, Finnish does not need to repeat it.

English does something similar sometimes:

• I like the red shoes, not the black ones.

In Finnish, the adjective can stand on its own when the noun is understood.

Because this sentence uses the pattern tehdä jostakin jotakin, where the result is expressed as a nominative form like mukavammat.

So here:

• niistä = the starting point
• mukavammat = the result

Mukavampia would not be the natural choice in this sentence.

Mukavammiksi belongs to a different but related structure:

tehdä jokin joksikin

For example, you could also say:

Uudet linssit tekevät ne mukavammiksi.

That also means The new lenses make them more comfortable, but the grammar is built differently.

So:

• tekevät niistä mukavammat = one correct pattern
• tekevät ne mukavammiksi = another correct pattern

No, this is a very normal, neutral word order.

The sentence has two clauses:

• Nämä kehykset ovat kevyet
• ja uudet linssit tekevät niistä mukavammat

This is straightforward and natural Finnish: subject + verb + complement.

Finnish word order is more flexible than English word order, but this version sounds neutral and is a good model for learners.