Optikko suositteli silmätippoja, jotta silmät eivät väsyisi niin nopeasti.

Jotta introduces a purpose clause. It means so that or in order that.

So this part:

jotta silmät eivät väsyisi niin nopeasti

means something like:

so that the eyes would not get tired so quickly

It tells you the purpose of the recommendation.

After jotta, Finnish very often uses the conditional to express an intended result or purpose.

So:

• väsyvät = get tired / are getting tired
• väsyisi = would get tired

In English, would is often natural in this kind of sentence:

• The optician recommended eye drops so that the eyes would not get tired so quickly.

So this is not mainly about uncertainty. It is the normal way Finnish often builds this kind of so that sentence.

Because the subject is plural: silmät = eyes.

Finnish negation uses a special negative verb that changes according to the subject:

• en = I do not
• et = you do not
• ei = he/she/it does not
• emme = we do not
• ette = you plural do not
• eivät = they do not

Since silmät is plural, Finnish uses eivät.

So:

• silmä ei väsyisi = the eye would not get tired
• silmät eivät väsyisi = the eyes would not get tired

Suositteli is the past tense of suositella, which means to recommend.

• suositella = to recommend
• suosittelee = recommends
• suositteli = recommended

So Optikko suositteli... means The optician recommended...

Silmätippoja is the partitive plural of silmätipat.

Base forms:

• silmätippa = an eye drop
• silmätipat = eye drops
• silmätippoja = eye drops, in the partitive plural

It is used here because the object of suositella is commonly in the partitive when talking about something indefinite or non-total, like recommending some medicine or some kind of product rather than a specific complete set.

So suositteli silmätippoja is a very natural way to say:

recommended eye drops

not:

recommended the specific eye drops as a complete finished object

Because Finnish normally uses the plural when talking about both eyes.

• silmä = eye
• silmät = eyes

In this sentence, the meaning is about your eyes in general, usually both of them, so plural is the natural choice.

English does the same here: the eyes.

Niin nopeasti means so quickly or that quickly.

• niin = so
• nopeasti = quickly

In this sentence it means the eyes would not get tired so quickly.

It adds degree: not just not get tired, but not get tired as fast as before / not get tired so fast.

It is basically normal Finnish word order.

Here is the structure:

• silmät = subject
• eivät = negative verb
• väsyisi = main verb
• niin nopeasti = adverb phrase

So the pattern is:

subject + negative verb + main verb + adverbial

That is a very standard order in Finnish.

You can move parts around for emphasis in other contexts, but this version is neutral and natural.

Finnish does not normally use articles the way English does.

So:

• optikko can mean the optician or an optician
• silmätippoja can mean eye drops
• silmät can mean the eyes

The exact meaning comes from context.

In this sentence, English naturally uses the optician because it sounds like a specific optician the person visited, but Finnish does not need a separate word for the.

Yes. A common alternative is to use etteivät instead of jotta ... eivät:

Optikko suositteli silmätippoja, etteivät silmät väsyisi niin nopeasti.

This still means roughly the same thing.

Very roughly:

• jotta silmät eivät väsyisi = so that the eyes would not get tired
• etteivät silmät väsyisi = another compact way to express the same idea

The version with jotta is very clear for learners because the purpose relationship is easy to see.

The basic verb is väsyä, which means to get tired.

Some useful forms:

• väsyä = to get tired
• väsyy = gets tired
• väsyi = got tired
• väsyisi = would get tired

So in the sentence:

silmät eivät väsyisi

the idea is:

the eyes would not get tired

or more naturally in context:

the eyes would not tire so quickly