Learn Hardcore Finnish: Käyn kolme kertaa viikossa puistossa treenaamassa juoksua jalkakäytävän reunaa pitkin. - I go three times a week to the park to train running along the edge of the sidewalk.

Menen puistoon treenaamaan juoksua...
that would describe the action of going there right now / at this moment, not the general habit.
Juoksen kolme kertaa viikossa puistossa...
would focus more on the running itself, without the nuance of "I go to the park (as a place I visit)".

So käyn emphasizes the habitual visits to the park.

What exactly does the structure käydä + verb in -massa mean?

The pattern käydä + (3rd infinitive in inessive: -massa / -mässä) means:

to go (somewhere) to do something (and then come back)

Some examples:

Käyn kaupassa ostamassa ruokaa.
I go to the store to buy food.
Käyn salilla treenaamassa.
I go to the gym to work out.

In your sentence:

käyn ... puistossa treenaamassa juoksua
= I go to the park to train running (as a regular activity).

So the -massa form expresses being engaged in that activity at the place you go to.

What grammatical form is treenaamassa?

Treenaamassa is the 3rd infinitive in the inessive case of the verb treenata.

Verb stem: treenaa-
3rd infinitive base: treenaama-
Inessive ending: -ssa / -ssä

→ treenaama + ssa = treenaamassa

This 3rd infinitive in inessive (-massa / -mässä) is used:

after verbs like käydä to show doing an activity at a place
- käyn uimassa = I go (somewhere) to swim
- käyn treenaamassa = I go (somewhere) to train / work out

So grammatically, it is an infinitive form that behaves like a noun in the inessive case, but functionally it means "while doing / to do (activity at that place)".

Why is juoksua in the partitive case here?

Juoksua is the partitive of juoksu (running).

Here it is:

the object/complement of treenaamassa (training what? → running)
an unbounded / indefinite activity, not a single, completed event

In Finnish:

Verbs like treenata / harjoitella often take the partitive when you’re practising a skill / activity in general:
- treenaan pianonsoittoa = I practice piano playing
- harjoittelen suomea = I practice Finnish
- treenaan juoksua = I practice running

The partitive here signals ongoing, not fully delimited activity, rather than one clearly completed object.

How would the meaning change if we said juoksun instead of juoksua?

Juoksun is the genitive form of juoksu.

treenaan juoksua
= I train running in general (the skill, the activity)
treenaan juoksun
could suggest training for a specific run / race / event, something more bounded and identifiable (for example, a particular competition).

In everyday speech, juoksua is the normal, natural choice when you mean running as a general sport or skill. Juoksun would sound more specific and less common in this generic context.

Could we say treenaan juoksua instead of käyn treenaamassa juoksua?

Yes, but the nuance changes.

TreenaAN juoksua kolme kertaa viikossa puistossa.
= I train running three times a week in the park.
Focus: the training activity itself.
Käyn treenaamassa juoksua...
= I go to train running…
Focus: the regular visits to the place (the park) to do that activity.

Both are correct, natural Finnish.
The käyn + -massa version highlights the habitual going somewhere, while treenaan is a straightforward present tense about what you do.

Why is it kolme kertaa, not kolme kerta?

Kerta (time, occurrence) behaves like a normal noun with numbers:

Singular nominative: kerta
Singular partitive: kertaa

After a numeral (2, 3, 4, …), non-human countable nouns usually appear in the partitive singular in this kind of expression:

kaksi kertaa = two times
kolme kertaa = three times
viisi kertaa = five times

So kolme kertaa is number (3) + partitive singular of kerta. The form kolme kerta is simply incorrect.

Why is it kolme kertaa viikossa, and not kolme kertaa viikko?

The phrase kolme kertaa viikossa literally means:

three times in a week / per week

Here:

kertaa = partitive singular of kerta (times)
viikossa = inessive case of viikko → "in a week"

Finnish expresses "X times per week" as:

X kertaa viikossa
- kerran viikossa = once a week
- kolme kertaa viikossa = three times a week

Using viikko in the basic form (kolme kertaa viikko) would be ungrammatical here; the inessive viikossa is required in this time-frequency expression.

Why is it puistossa and not puistoon?

The difference:

puistossa = in the park (inessive case: inside/at a location)
puistoon = into the park (illative case: movement into)

In your sentence:

Käyn ... puistossa treenaamassa...
= I go to the park and am in the park when I train.

With käydä + -massa, the place where you do the activity is normally in the inessive:

käyn kaupassa ostamassa maitoa = in the store
käyn salilla treenaamassa = in/at the gym
käyn puistossa treenaamassa = in the park

If you used puistoon, it would emphasize movement into the park, but with käydä plus an activity, puistossa is the standard, natural choice.

Can we change the word order, for example: Käyn puistossa kolme kertaa viikossa treenaamassa juoksua...?

Yes, Finnish word order is quite flexible. All of these are acceptable:

Käyn kolme kertaa viikossa puistossa treenaamassa juoksua...
Käyn puistossa kolme kertaa viikossa treenaamassa juoksua...
Käyn treenaamassa juoksua puistossa kolme kertaa viikossa...

The basic meaning stays the same. Word order mainly affects:

emphasis (what you want to highlight first)
rhythm / style

The original order (kolme kertaa viikossa early in the sentence) emphasizes the frequency first. Putting puistossa earlier emphasizes the place first.

What does jalkakäytävän reunaa pitkin literally mean, and why these forms?

Literally:

jalkakäytävän = of the sidewalk (genitive)
reunaa = the edge (partitive)
pitkin = along

So:

jalkakäytävän reunaa pitkin
≈ along the edge of the sidewalk

Case usage:

jalkakäytävän in genitive modifies reuna → "the edge of the sidewalk"
reunaa in partitive is required by pitkin (which takes its complement in partitive)
pitkin itself is a postposition meaning along, following the length of

So it is a typical structure:

[Noun-GENITIVE] + reuna (in PARTITIVE) + pitkin
= along the edge of [Noun].

Why is jalkakäytävän in the genitive case?

Jalkakäytävän (of the sidewalk) is in the genitive because it possesses or specifies the noun reuna (edge).

Pattern:

[Possessor / specifying noun in genitive] + [head noun]
- talon ovi = the door of the house
- kaupungin keskusta = the center of the city
- jalkakäytävän reuna = the edge of the sidewalk

In your phrase, reuna is in the partitive (reunaa) because of pitkin, but the relationship "edge of the sidewalk" is still expressed by jalkakäytävän in genitive.

Is pitkin a preposition or a postposition, and which case does it require?

Pitkin is usually a postposition in Finnish:

It follows the word it governs:
- jalkakäytävän reunaa pitkin = along the edge of the sidewalk

It requires the noun it governs to be in the partitive case:

tietä pitkin = along the road (tie → tietä)
rantaa pitkin = along the shore (ranta → rantaa)
katua pitkin = along the street (katu → katua)

So reunaa is in the partitive specifically because of pitkin.

Could we drop parts like juoksua or jalkakäytävän reunaa pitkin and still be correct?

Yes, you can shorten the sentence while staying grammatical; you just lose detail.

For example:

Käyn kolme kertaa viikossa puistossa treenaamassa.
= I go three times a week to the park to train (it’s implied what you train).
Käyn kolme kertaa viikossa puistossa treenaamassa juoksua.
(no jalkakäytävän reunaa pitkin)
= I go three times a week to the park to train running (no information about the edge of the sidewalk).

Each element you remove simply removes one piece of information, but the core grammar käyn ... puistossa treenaamassa remains perfectly correct.

How would you say this in the past: “I went three times a week to the park…”?

You mainly change käyn to its past tense kävin:

Kävin kolme kertaa viikossa puistossa treenaamassa juoksua jalkakäytävän reunaa pitkin.

This means:

I used to go / went three times a week to the park to train running along the edge of the sidewalk.

The rest of the sentence stays the same, because Finnish does not mark tense on adverbials like kolme kertaa viikossa, only on the verb kävin.