Varasin ajan optikolle, koska haluan käydä näöntarkastuksessa ensi viikolla.

Varasin is the 1st person singular past tense of varata (to book / reserve).

• varata = to book
• varasin = I booked

So the sentence starts in the past: the speaker already made the appointment.

In the expression varata aika / varata ajan, aika means an appointment.

Here it appears as ajan, which is the singular form used as the object in this sentence. With a completed action in the past, Finnish often uses the total object, and in the singular that often looks like the genitive form:

• aika = time / appointment
• ajan = appointment (object form here)

So varasin ajan means I booked an appointment.

This is a very common fixed expression in Finnish:

• varata aika lääkärille
• varata aika hammaslääkärille
• varata aika optikolle

Optikolle is in the allative case (-lle), which often means to, onto, or for/to someone.

Here it means the appointment was booked with / to the optician:

• optikko = optician
• optikolle = to/with the optician

In Finnish, when talking about booking an appointment with a professional, the allative is very common:

• varata aika lääkärille = book an appointment with the doctor
• varata aika kampaajalle = book an appointment with the hairdresser

So optikolle is the natural form here.

Koska means because.

It introduces the reason:

• Varasin ajan optikolle = I booked an appointment with the optician
• koska haluan käydä näöntarkastuksessa ensi viikolla = because I want to go for an eye test next week

So koska connects the action and the reason for it.

Both käydä and mennä can mean to go, but they are not used in exactly the same way.

Here käydä is very natural because it often means:

• to go somewhere and come back
• to go for a visit
• to have something done

So:

• käydä lääkärissä = go to the doctor
• käydä hammastarkastuksessa = go for a dental check-up
• käydä näöntarkastuksessa = go for an eye test

Using mennä would focus more simply on movement to the place. Using käydä sounds more idiomatic here because the speaker means to attend the examination.

Näöntarkastuksessa is in the inessive case (-ssa / -ssä), which often means in.

But with words like examinations, appointments, and activities, Finnish often uses the inessive where English uses something like at, for, or no preposition at all.

• näöntarkastus = eye test / vision test
• näöntarkastuksessa = in/at a vision test

After käydä, this pattern is very common:

• käydä tarkastuksessa = go for a check-up
• käydä tutkimuksessa = go for an examination
• käydä näöntarkastuksessa = go for an eye test

So even though it literally looks like in the eye test, the natural English meaning is go for an eye test.

It is a compound word:

• näkö = sight / vision
• näön = of vision / vision’s
• tarkastus = inspection / check / examination

So:

• näöntarkastus = vision check, eye test

The first part changes to näön because Finnish compounds often use a linking form related to the genitive:

• näkö → näön-

Then the whole word gets the case ending:

• näöntarkastus
• näöntarkastuksessa

This kind of long compound is extremely common in Finnish.

Ensi viikolla uses the adessive case (-lla / -llä), which is the normal case for many expressions of time in Finnish.

• ensi viikolla = next week
• maanantaina = on Monday
• kesällä = in summer

So this is just the standard Finnish way to say when something happens.

By contrast, viikossa usually means in a week in the sense of within one week or after a week’s time, not next week.

Compare:

• Käyn siellä ensi viikolla. = I’ll go there next week.
• Teen sen viikossa. = I’ll do it in a week.

Because the two verbs refer to different times:

• Varasin = past action: the booking already happened.
• haluan = present feeling/desire: I want to do this.
• käydä ... ensi viikolla = the visit is in the future.

So the sentence mixes time naturally:

1. I booked the appointment earlier.
2. Right now, I want to have the eye test.
3. The eye test will happen next week.

This kind of tense combination is very normal in both Finnish and English.

Not always. The basic meaning of aika is time. But in certain expressions it also means appointment.

Examples:

• Minulla ei ole aikaa. = I don’t have time.
• Minulla on aika lääkärille. = I have a doctor’s appointment.
• Varasin ajan optikolle. = I booked an appointment with the optician.

So the meaning depends on context. In healthcare and service situations, aika very often means appointment.

Yes, Finnish word order is fairly flexible, although this version is very neutral and natural:

• Varasin ajan optikolle, koska haluan käydä näöntarkastuksessa ensi viikolla.

You could move parts around for emphasis, for example:

• Ensi viikolla haluan käydä näöntarkastuksessa, joten varasin ajan optikolle.
• Koska haluan käydä näöntarkastuksessa ensi viikolla, varasin ajan optikolle.

The original sentence sounds like a normal, straightforward statement.

Possibly, depending on context, but näöntarkastus is a very standard word for eye test / vision test.

• näöntarkastus focuses on vision
• silmätarkastus could sound more like an eye examination in a broader sense

If you mean checking how well you see, especially at the optician, näöntarkastus is the most natural choice.

Finnish does not have articles like English a/an and the.

So:

• Varasin ajan can mean I booked an appointment or, depending on context, I booked the appointment.

The exact meaning is understood from context rather than from an article. That is completely normal in Finnish.