Learn Hardcore Finnish: Valmentaja yllättyi, kun olin kuitenkin jaksanut juosta loppuun asti. - The coach was surprised when I had nevertheless managed to run all the way to the end.

Valmentaja yllättyi (the coach got surprised) happens after or as a reaction to the fact that
olin jaksanut juosta loppuun asti (I had managed to run all the way to the end).

Using the past perfect in the kun‑clause makes the order of events clear: first you managed to run to the end, then the coach became surprised.

You could say:

Valmentaja yllättyi, kun jaksoin juosta loppuun asti.

That is also grammatical, but it sounds a bit more like the two events are on the same time line, and it gives slightly less emphasis to the idea that the running was fully completed before the surprise. The original version underlines that your effort was already fully accomplished when the coach reacted.

What exactly does jaksanut juosta mean, and why is juosta in the infinitive?

The verb jaksaa means something like to have the energy / strength / stamina, or more generally to manage (to do something).

The pattern is:

jaksaa + basic infinitive (1st infinitive)

So:

jaksaa juosta = to have the energy to run, to be able to keep running
olin jaksanut juosta = I had had the energy to run / I had managed to run

Here, juosta is in its basic infinitive form because it is the complement of jaksaa. Jaksaa is the verb that is conjugated for person and tense; juosta just tells you what you had the energy to do.

Compare:

Haluan juosta. – I want to run.
Yritin juosta. – I tried to run.
En jaksanut juosta. – I did not have the energy to run.

In all of these, the second verb is in the infinitive for the same reason.

What does kuitenkin mean here, and where can it go in the sentence?

Kuitenkin is an adverb that usually corresponds to English however, nevertheless, still, or after all, depending on context.

In this sentence it adds the idea that, despite expectations or difficulties, you still managed:

olin kuitenkin jaksanut juosta loppuun asti
≈ I had still / nevertheless managed to run all the way to the end.

About position:

The neutral, very common position is exactly as in the example: after the auxiliary verb olin and before the participle:
olin kuitenkin jaksanut...

Other possible (and grammatical) positions:

Kuitenkin olin jaksanut juosta loppuun asti.
Slightly more emphasis on kuitenkin itself; stylistically a bit more marked.
Olin jaksanut kuitenkin juosta loppuun asti.
Also possible, but it can sound a bit clumsy; it may suggest emphasis on the running to the end as the thing you nevertheless managed.

So the given word order is natural and idiomatic, and matches how English often puts still or nevertheless right before the main verb phrase.

What does loppuun asti literally mean, and why are both words needed?

Breakdown:

loppu = end
loppuun = illative form of loppu, roughly into the end / to the end
asti = a postposition meaning until, up to, as far as

Together, loppuun asti means (all the way) until the end or right to the end.

The parts:

juosta loppuun can already mean to run to the end, or to run something until it is finished / used up.
asti adds explicitness and often a feeling of extent: all the way up to that endpoint.

Very similar combinations exist in everyday Finnish:

aamuun asti – until morning
kouluun asti – all the way to school
tähän asti – up to this point / until now

You could also say loppuun saakka; asti and saakka are near-synonyms here. The main idea is: loppuun asti strongly emphasizes completion: you did not stop or give up before the end.

Why is the conjunction kun used here, and how is it different from koska?

Kun is a very flexible conjunction. Its main uses are:

Temporal: when, as
- Kun tulin kotiin, söin. – When I came home, I ate.
Causal (especially in speech): since, as, because
- Kun olet jo täällä, tule sisään. – Since you are already here, come in.

In Valmentaja yllättyi, kun olin kuitenkin jaksanut juosta loppuun asti, kun is primarily temporal:

The coach got surprised when he realized you had managed to run all the way to the end.

You could also understand a light causal nuance (he was surprised because you had managed), but the basic reading is time‑based.

Koska is more clearly and purely causal:

Valmentaja yllättyi, koska olin jaksanut juosta loppuun asti.
– The coach was surprised because I had managed to run to the end.

So:

kun here sounds more like when, giving the time/context of the surprise.
koska would foreground the cause, like a clear-cut explanation.

Why is there a comma before kun in Finnish, even though in English it might be optional?

In Finnish, the rule for commas before subordinate clauses is quite strict:

A subordinate clause introduced by kun, että, jos, joskus, vaikka, etc., is normally separated from the main clause by a comma.

So in:

Valmentaja yllättyi, kun olin kuitenkin jaksanut juosta loppuun asti.

Valmentaja yllättyi is the main clause, and kun olin kuitenkin jaksanut juosta loppuun asti is a subordinate clause, so there must be a comma between them.

This is more mechanical than in English. In English you might write:

The coach was surprised when I had still managed to run to the end. (no comma)
The coach was surprised, when I had still managed to run to the end. (less common)

In Finnish the comma is not really a stylistic choice here; it is simply required by the standard punctuation rules.

What is the difference between yllättyi, yllätti, and oli yllättynyt?

All are related to the idea of being surprised, but they work differently.

yllättyi
- From verb yllättyä = to become surprised.
- Intransitive: no direct object.
- Valmentaja yllättyi. – The coach became surprised / was taken by surprise.
  Focus: the moment of becoming surprised.
yllätti
- From verb yllättää = to surprise (someone).
- Transitive: takes a direct object.
- Valmentaja yllätti minut. – The coach surprised me.
  Here the coach is the one causing the surprise.
oli yllättynyt
- olla
  - adjectival participle yllättynyt.
- Describes a state.
- Valmentaja oli yllättynyt. – The coach was surprised.
  Focus: the state of being surprised, not the moment when it started.

So in your sentence, yllättyi nicely captures the coach’s reaction as a sudden response to what you managed to do.

How do you know whether valmentaja means the coach or a coach, since there is no article?

Finnish does not have articles like a or the. The noun valmentaja is just coach in a bare form.

The specificity (whether it is the coach or a coach) comes from context:

If the people in the conversation already know which coach you are talking about (your team’s coach, for example), natural English will use the coach.
If this is the first time a coach is mentioned and the identity is not known or important, English might use a coach.

So:

Valmentaja yllättyi...
Normally in a realistic context would be translated as The coach was surprised..., because you usually have some particular coach in mind.

This is a general feature of Finnish, not specific to this sentence.

Could the clause starting with kun be put first, and would that change the meaning?

Yes, you can put the kun‑clause first:

Kun olin kuitenkin jaksanut juosta loppuun asti, valmentaja yllättyi.

This is completely grammatical and natural.

Meaning-wise, it is still:

When I had nevertheless managed to run to the end, the coach was surprised.

The basic content is the same, but the emphasis shifts slightly:

Original order (Valmentaja yllättyi, kun...)
– Slightly more focus on the coach’s reaction.
Reversed order (Kun olin kuitenkin jaksanut..., valmentaja yllättyi.)
– Slightly more focus on the condition / background situation; it can sound a bit more narrative, building up to the reaction.

This kind of clause reordering is very common and flexible in Finnish.

Why is it juosta loppuun asti, not something like juoksemaan loppuun asti?

The choice of form depends on the main verb’s rections (what kind of complement it requires).

jaksaa typically takes the first infinitive (basic form):
jaksaa + V1
→ jaksaa juosta, jaksaa tehdä, jaksaa lukea, etc.

So:

olin jaksanut juosta loppuun asti is the normal, correct pattern.

A structure like jaksoin juoksemaan is not correct here, because jaksaa does not take the maan-form infinitive (the so‑called third infinitive illative).

By contrast, some other verbs do need the maan / mään form:

mennä juoksemaan – go (in order) to run
aloittaa juoksemaan – (less common; more natural is aloittaa juoksemisen or alkaa juosta, but the point is that some verbs can take this form)

So the form juosta is used because jaksaa belongs to the group of verbs that govern the basic infinitive.