En osannut päättää nopeasti, joten kysyin tarjoilijalta suositusta.

Because Finnish forms negation differently from English.

In a negative sentence, Finnish uses a special negative verb, and that verb is the one that gets conjugated for person:

• en = I do not / did not
• et = you do not / did not
• ei = he/she/it does not / did not
• etc.

In the past negative, the main verb is not in the normal past tense form. Instead, it appears in a form like osannut.

So:

• positive past: osasin
• negative past: en osannut

That is why en osasin is incorrect.

Because Finnish often leaves out subject pronouns when they are already clear from the verb form.

Here, en already tells you the subject is I, and kysyin also shows I.

So:

• Minä en osannut päättää... = possible, but more explicit
• En osannut päättää... = more natural in many everyday contexts

Finnish does this very often.

That is its basic meaning, yes. But in Finnish, osata + infinitive can also be used in situations where English would say can, be able to, or even manage to.

So in this sentence, en osannut päättää is not really about learned skill in a strict sense. It means something like:

• I couldn't decide
• I wasn't able to decide
• I couldn't make up my mind

It is a very natural Finnish way to express that idea.

Because after osata, Finnish normally uses the first infinitive of the next verb.

So the pattern is:

• osata tehdä
• osata puhua
• osata päättää

Here:

• osannut = was able / knew how
• päättää = to decide

So en osannut päättää follows a very common structure: osata + infinitive.

Yes, en voinut päättää could also work, but the nuance is a little different.

• en osannut päättää suggests I couldn't make up my mind or I didn't manage to decide
• en voinut päättää suggests more literally I could not decide, often because something prevented it, or simply as a more neutral statement of inability

In many contexts the two are close, but en osannut päättää sounds especially natural when someone is hesitating between options.

Because nopeasti is an adverb, and it describes how the deciding happened.

• nopea = fast, quick (adjective)
• nopeasti = quickly (adverb)

Finnish often forms adverbs from adjectives by adding -sti:

• hidas → hitaasti
• varma → varmasti
• nopea → nopeasti

So päättää nopeasti means to decide quickly.

Joten means so, therefore, or as a result.

It connects two clauses where the second clause is the consequence of the first one:

• En osannut päättää nopeasti, joten kysyin...
• I couldn't decide quickly, so I asked...

It is a very common conjunction for linking cause and result.

Because Finnish uses the pattern:

kysyä joltakulta jotakin

That means:

• ask someone for something
• literally, ask something from someone

So:

• tarjoilija = waiter / waitress / server
• tarjoilijalta = from the waiter/server

The ending -lta / -ltä is the ablative case, which often has the idea of from.

Compare:

• kysyin opettajalta = I asked the teacher
• kysyin ystävältäni = I asked my friend

Because with kysyä, the thing being asked for is very often in the partitive.

So:

• kysyä suositusta
• kysyä neuvoa
• kysyä apua
• kysyä lupaa

Here suositusta is the singular partitive of suositus.

The partitive fits well because the speaker is asking for a recommendation / some recommendation, not referring to a specific fully identified recommendation already known to both speakers.

For a learner, the safest thing is to remember this pattern:

kysyä joltakulta jotakin

and very often that jotakin part is in the partitive.

It can mean either. Finnish job titles are usually not marked for gender.

So tarjoilija can refer to:

• a waiter
• a waitress
• a server

Usually the context tells you who is meant. Finnish does not need separate everyday words the way English traditionally does.

The sentence uses a very neutral, natural word order.

• En osannut päättää nopeasti, joten kysyin tarjoilijalta suositusta.

Finnish word order is more flexible than English, but changing it usually changes the focus or emphasis, not the basic meaning.

For example, moving nopeasti or putting something first could make that part sound more emphasized. But the version you have is the most straightforward, neutral way to say it.