Learn Hardcore Finnish: Matka tuntuu helpolta, kun juna on nopea. - The trip feels easy when the train is fast.

Because the grammatical subject is matka (the trip/journey), which is singular. The verb tuntua (to feel/seem) agrees with its subject in person/number, so matka tuntuu = the trip feels/seems.

What role does matka have here—subject or object?

Matka is the subject. In Finnish, the thing that “feels” a certain way is commonly the subject with tuntua:

Matka tuntuu… = The trip feels…
There isn’t a direct object in this sentence.

Why is helpolta in that form, and what case is it?

Helpolta is the adjective helppo in the ablative case (-lta/-ltä). This is a standard construction with tuntua:

tuntua joltakin = to feel/seem like something

So tuntuu helpolta literally follows the pattern feels (like) easy.

Why isn’t it Matka tuntuu helppo (plain nominative) instead of helpolta?

Finnish doesn’t normally use a plain nominative adjective as the complement of tuntua. With olla you’d use nominative:

Matka on helppo. = The trip is easy. (more objective statement)

With tuntua, Finnish typically uses the joltakin pattern:

Matka tuntuu helpolta. = The trip feels easy. (speaker’s impression)

Could I say Matka tuntuu helpoksi instead of helpolta?

Usually helpolta is the natural choice for “feels/seems easy.”
Helpoksi (translative -ksi) can exist, but it more easily suggests “seems/turns out (to be) easy” in a classificatory or result-like sense, depending on context. For a straightforward “feels easy,” helpolta is the safe default.

What does kun mean here—“when” or “because”?

Kun can mean both, depending on context. Here it can be understood as:

when / as: the trip feels easy when the train is fast (general condition)
because: the trip feels easy because the train is fast (reason)

If you want an unambiguously causal “because,” koska is often used:

…koska juna on nopea.

Why is there a comma before kun?

Finnish normally uses a comma before a subordinate clause introduced by kun, koska, että, etc. So:

Matka tuntuu helpolta, kun …
is standard punctuation.

Why is it juna on nopea (adjective) and not an adverb like “fast”?

Because on is the verb olla (to be), which links the subject to a predicate adjective:

juna on nopea = the train is fast (adjective describing the train)

An adverb would describe an action, e.g.:

Juna kulkee nopeasti. = The train moves fast.

Why is nopea in the nominative, not partitive (nopeaa)?

In an affirmative copula sentence like this, the predicate adjective is typically nominative:

Juna on nopea.

Partitive (nopeaa) would appear in contexts like negation or certain “partial/indefinite” meanings:

Juna ei ole nopea. (often nominative in modern Finnish)
Juna ei ole nopea/nopeaa. (variation exists; usage depends on dialect/style and nuance)

For a basic positive statement, nopea is the expected form.

Why is the verb on present tense? Could it be oli?

Present tense (on) makes it a general statement or present-time condition:

…kun juna on nopea. = …when/because the train is fast.

Past tense (oli) would shift the condition to the past:

Matka tuntui helpolta, kun juna oli nopea. = The trip felt easy when/because the train was fast.

Does Finnish need a word for “the” in Matka / juna?

No. Finnish has no articles (the / a). Context supplies definiteness:

Matka can be a trip or the trip
juna can be a train or the train

Is the word order fixed? Could I move the kun-clause?

The meaning stays mostly the same, but focus changes. You can front the clause:

Kun juna on nopea, matka tuntuu helpolta.
This puts the condition/reason first, which can sound slightly more explanatory or structured.