Optikko auttoi minua valitsemaan kehykset, joihin uudet linssit sopivat paremmin.

Because minua is the form required by the verb auttaa when it takes a person as its object.

• minä = I (subject form)
• minua = me (partitive object form)

In Finnish, auttaa normally takes the person being helped in the partitive:

• Optikko auttoi minua = The optician helped me

So this is not the subject of the sentence; it is the person receiving the help.

After auttaa, Finnish often uses the -maan / -mään form of the verb, called the illative of the MA-infinitive.

So:

• auttoi minua valitsemaan = helped me choose

This structure means something like helped me in the process of choosing or helped me to choose.

Compare:

• valita = the basic infinitive, to choose
• valitsemaan = to choose / into choosing in this specific construction after verbs like mennä, lähteä, jäädä, auttaa, etc.

So auttoi minua valitsemaan is the natural Finnish structure here.

Kehykset is the object of valitsemaan.

Here it is a total object in the plural, and in modern Finnish that looks the same as the nominative plural form.

So:

• kehykset = the frames

Why a total object? Because the action is understood as complete: the speaker is choosing the frames, not just doing some indefinite choosing of frames.

Compare the idea:

• valita kehykset = choose the frames
• valita kehyksiä = choose frames / be choosing some frames (more indefinite or incomplete)

So kehykset fits the idea of selecting a definite pair/set of frames.

Because the verb sopia requires a specific case here.

In this sentence, the idea is:

• linssit sopivat kehyksiin = the lenses fit into the frames

Since kehykset would be kehyksiin after sopia, the relative pronoun must match that same case:

• joihin = into which / which ... into

So:

• kehykset, joihin uudet linssit sopivat paremmin = the frames into which the new lenses fit better

If you used jotka, that would be nominative and would not fit the grammar of sopia johonkin.

Joihin is the plural illative form of the relative pronoun joka.

A useful pattern is:

• joka = who/which/that
• joihin = into which / which ... into

It refers back to kehykset.

You can think of the structure like this:

• kehykset
• uudet linssit sopivat kehyksiin
• combine them: kehykset, joihin uudet linssit sopivat

So joihin is doing the job of which in a relative clause, while also carrying the case ending required by sopia.

Because uudet linssit is the subject of the relative clause.

In the clause:

• uudet linssit sopivat paremmin

the thing doing the fitting is uudet linssit (the new lenses), so it is the subject and appears in the nominative plural.

Breakdown:

• uudet = new
• linssit = lenses
• together: uudet linssit = the new lenses

They are plural, so the verb is plural too: sopivat.

Because its subject, uudet linssit, is plural.

Finnish verbs agree with the subject in number:

• linssi sopii = the lens fits
• linssit sopivat = the lenses fit

So sopivat is simply the 3rd person plural form of sopia.

Here sopia means to fit.

That verb can have a few related meanings in Finnish, depending on context:

• sopia johonkin = fit into something
• sopia jollekulle = suit someone
• sopia jostakin = agree on something

In this sentence, the physical meaning is intended:

• linssit sopivat kehyksiin = the lenses fit into the frames

So this is about compatibility of the lenses and the frames.

Paremmin means better. It is the comparative form of the adverb hyvin (well).

• hyvin = well
• paremmin = better

In this sentence, the comparison is implied rather than stated explicitly. It likely means:

• the new lenses fit these frames better than some other frames
• or better than the speaker’s current frames
• or better than another available option

Finnish often leaves the second half of the comparison unstated when it is clear from context.

Because joihin uudet linssit sopivat paremmin is a relative clause modifying kehykset.

Finnish normally uses a comma before a relative clause:

• kehykset, joihin...

This is similar to English punctuation in many cases, though Finnish is generally more consistent about marking subordinate clauses with commas.

So the comma helps show that the sentence has two parts:

1. Optikko auttoi minua valitsemaan kehykset
2. joihin uudet linssit sopivat paremmin

Not completely. Finnish word order is fairly flexible, though the given version is natural and clear.

The neutral order here is:

• Optikko = subject
• auttoi = verb
• minua = object of auttoi
• valitsemaan kehykset = infinitive phrase
• joihin... = relative clause

You could rearrange parts for emphasis, but the original sentence sounds natural and unmarked.

For example, moving uudet linssit earlier inside the relative clause is possible only within normal Finnish rules, but the standard version here is already the one most learners should aim for.

Finnish has no articles like English a/an and the.

So:

• optikko can mean an optician or the optician
• kehykset can mean frames or the frames
• uudet linssit can mean new lenses or the new lenses

The exact meaning depends on context.

In this sentence, English usually uses the optician, the frames, and the new lenses because the situation sounds specific, but Finnish does not need separate article words to show that.

Yes, kehykset is the normal word for eyeglass frames, and it is usually used in the plural.

• kehys = frame
• kehykset = frames

For eyeglasses, Finnish commonly treats the frame structure as a plural item, much like English often says glasses or frames.

So in this context, kehykset is the natural choice. Using singular kehys would usually sound less natural unless you were talking about one frame as a single object in some special technical sense.