Learn Hardcore Finnish: Kävelen polkua pitkin, kunnes näen järven. - I walk along the path until I see the lake.

Kävelen is the present tense, 1st person singular form of kävellä (to walk): I walk / I am walking.
In Finnish, the present tense is also commonly used for actions that are ongoing or about to happen (and even for future-like meaning when the context is clear).

kävelin = past: I walked / I was walking
kävelisin = conditional: I would walk

Do I need to say minä kävelen, or is kävelen enough?

Kävelen is usually enough because the verb ending -n already shows “I”.
You add minä mainly for emphasis or contrast (like “I am the one walking, not someone else”).

What case is polkua, and why is it in that case?

Polkua is partitive singular of polku (path).
With pitkin (along), Finnish typically uses the partitive:

polkua pitkin = along the path

This is a set pattern: pitkin + partitive (very common with routes/areas).

What is pitkin—a preposition or a postposition—and why is it placed after the noun?

Pitkin can function as either, but here it’s a postposition, placed after the noun phrase:

polkua pitkin = along the path

You can also see the preposition-like order:

pitkin polkua (also correct)

Both are common; the key grammar point is still that the noun is typically partitive.

Why is there a comma before kunnes?

Because kunnes introduces a subordinate clause (kunnes näen järven). In Finnish, it’s standard to separate the main clause and subordinate clause with a comma:

Kävelen polkua pitkin, kunnes ...

How does kunnes work here, and how is it different from kun?

kunnes = until (an endpoint is reached)
kun = when (time relation, not necessarily “until”)

So:

kunnes näen järven = until I see the lake (walking continues up to that point)
kun näen järven = when I see the lake (focuses on the moment, not the “up to” duration)

Why is it näen and not something like tulen näkemään for the future?

Finnish often uses the present tense for events that are in the future relative to now, especially in time clauses like this:

kunnes näen järven = until I (will) see the lake

A more explicitly “future-ish” phrasing like tulen näkemään (I will come to see) exists, but it often adds extra nuance (more deliberate, more emphatic, sometimes less natural for simple statements).

What’s going on with the verb näen—why does it look so different from the dictionary form nähdä?

This is normal Finnish verb stem behavior. nähdä is an irregular/high-frequency verb.
Its present tense forms include:

minä näen = I see
sinä näet = you see
hän näkee = he/she sees

So näen is simply the 1st person singular present form.

Why is it järven and not järvi or järveä?

Järven is the genitive/accusative-looking object form (often called the total object) of järvi (lake). It fits when the action is seen as reaching a clear result/endpoint: I see the lake (at that point).

If you said järveä (partitive), it would suggest a more ongoing/partial view:

näen järveä ≈ I can see (some of) the lake / I’m seeing the lake (as an ongoing experience)

In this sentence with kunnes (an endpoint), järven is very natural.

Does järven mean “the lake” or “a lake”? How do I know?

Finnish has no articles (a/the), so järven can correspond to either “a lake” or “the lake”.
Definiteness is inferred from context (shared knowledge, earlier mention, situation). The case ending here is about grammar (object type), not definiteness.

Could I change the word order, like Kävelen, kunnes näen järven, polkua pitkin?

Finnish word order is flexible, but not all rearrangements sound equally natural. The neutral, clear order is:

Kävelen polkua pitkin, kunnes näen järven.

You can move elements for emphasis, but separating polkua pitkin away from kävelen may feel heavier or more poetic/marked. Usually you’d keep the route expression close to the movement verb.

If I wanted to say “I walk along the path” more explicitly, is there a way?

You can’t mark the directly, but you can make it contextually definite, for example by specifying:

Kävelen tätä polkua pitkin... = I’m walking along this path...
Kävelen samaa polkua pitkin... = I’m walking along the same path...
Kävelen sitä polkua pitkin... = I’m walking along that (known) path...