Näöntarkastuksen jälkeen valitsen uudet kehykset, mutta vanhat linssit pitää vaihtaa.

Because jälkeen is a postposition meaning after, and postpositions usually require the noun before them to be in the genitive.

So:

• näöntarkastus = eye exam, vision test
• näöntarkastuksen jälkeen = after the eye exam

The -n ending on näöntarkastuksen is the genitive ending.

Yes. It is a compound:

• näkö = sight, vision
• tarkastus = inspection, check, examination

Together, näöntarkastus means a vision test or eye exam.

You may notice that the first part appears as näön- inside the compound. That is very common in Finnish compounds: the first element often appears in a linking form rather than its plain dictionary form.

Then the whole compound inflects normally:

• näöntarkastus = nominative
• näöntarkastuksen = genitive

Because the verb ending already tells you the subject.

• valitsen = I choose / I will choose

The ending -n marks first person singular, so minä is usually unnecessary unless you want emphasis or contrast.

For example:

• Valitsen uudet kehykset. = I choose / I’ll choose new frames.
• Minä valitsen uudet kehykset. = I am the one who chooses the new frames.

The dictionary form is valita, meaning to choose or to select.

Its present-tense stem is valitse-, so the forms are:

• valitsen = I choose
• valitset = you choose
• valitsee = he/she chooses

So valitsen is just the normal first-person singular present form of valita.

Also, in Finnish the present tense often covers near-future meaning too, depending on context. So here valitsen can mean either:

• I choose
• I will choose

It is a total object in the plural, and in Finnish a total object in the plural looks like the nominative plural.

So:

• uudet kehykset = the whole set of new frames, as a complete object

That is why you do not see a special accusative-looking form here.

A useful comparison:

• singular total object: uuden kehyksen
• plural total object: uudet kehykset

So valitsen uudet kehykset means choosing a complete, specific set of frames.

Because when talking about glasses, Finnish usually treats these as plural items:

• kehys = one frame
• kehykset = frames, eyeglass frames
• linssi = one lens
• linssit = lenses

In practice, glasses normally have a pair of lenses and a frame set, so plural forms are very natural here.

So:

• uudet kehykset = new frames
• vanhat linssit = old lenses

pitää + infinitive is a very common Finnish way to express necessity.

So:

• pitää vaihtaa = must change, have to change, need to replace

In this sentence, vanhat linssit pitää vaihtaa means that the old lenses need replacing.

A natural English translation is:

• the old lenses need to be replaced

It does not have to say who will do the replacing. Finnish often leaves that unspecified.

Because vanhat linssit is not really the doer of the action. It is the thing being replaced.

So even though vanhat linssit comes before pitää, it functions as the object of vaihtaa in terms of meaning.

This is why the sentence means:

• the old lenses need to be replaced

not:

• the old lenses must replace something

In this kind of necessity construction, the object often stays in the nominative form, especially with a plural total object like vanhat linssit.

Because Finnish normally uses a comma before mutta when it joins two independent clauses.

Here the two clauses are:

• Näöntarkastuksen jälkeen valitsen uudet kehykset
• mutta vanhat linssit pitää vaihtaa

So the comma is standard punctuation.

Yes. Finnish word order is fairly flexible.

The sentence begins with Näöntarkastuksen jälkeen to set the time frame first: after the eye exam.

You could also say things like:

• Valitsen uudet kehykset näöntarkastuksen jälkeen
• Valitsen näöntarkastuksen jälkeen uudet kehykset

These are also possible, though the original version is very natural because it puts the time expression first and gives the sentence a clear structure.

Because Finnish has no articles.

So Finnish does not have separate words corresponding directly to English a, an, or the.

That means:

• uudet kehykset can mean new frames or the new frames
• vanhat linssit can mean old lenses or the old lenses

The exact sense comes from context.

Yes. Finnish has several ways to express necessity.

For example:

• vanhat linssit pitää vaihtaa
• vanhat linssit täytyy vaihtaa
• vanhat linssit on vaihdettava

All of these mean roughly that the old lenses must be replaced.

The differences are mostly stylistic:

• pitää vaihtaa = very common, natural
• täytyy vaihtaa = also very common
• on vaihdettava = a bit more formal or written-sounding

In the original sentence, pitää vaihtaa is an everyday, natural choice.