Learn Hardcore Finnish: Minä kuljen usein polkua pitkin metsään. - I often walk along the path into the forest.

Polkua is in the partitive case (singular). The postposition pitkin (“along”) almost always requires the partitive form of the noun it governs.

polku = “path” (basic / nominative form)
polkua = “(of the) path” in partitive

So:

polkua pitkin ≈ “along the path”

You cannot say *polku pitkin; that would be ungrammatical.

Why is pitkin after polkua? In English “along” comes before “the path”.

In Finnish, pitkin is usually a postposition, not a preposition:

English: along the path (preposition + noun)
Typical Finnish: pitkin polkua or polkua pitkin (noun + postposition OR postposition + noun)

Both pitkin polkua and polkua pitkin are possible and natural.
Word order here is fairly flexible and often used for rhythm or emphasis, but the case (partitive) must stay the same:

Minä kuljen usein pitkin polkua metsään.
Minä kuljen usein polkua pitkin metsään.

Both mean essentially the same: “I often walk along the path into the forest.”

Why is it metsään and not metsässä? Aren’t both related to “forest”?

Yes, both come from metsä (“forest”), but they are different cases and express different relations:

metsässä = “in the forest” (inessive case → location inside)
metsään = “into the forest” (illative case → movement into)

Your sentence describes movement into the forest, not being already inside it:

Minä kuljen usein polkua pitkin metsään.
→ “I often walk along the path into the forest.”

If you said:

Minä kuljen usein polkua pitkin metsässä.

it would sound like you’re somehow walking along a path that is in the forest, but the idea of entering the forest is lost. The original feels more like you start outside and end up inside.

Can I leave out Minä and just say Kuljen usein polkua pitkin metsään?

Yes, and that is actually more typical Finnish.

The personal ending on the verb -n in kuljen already shows the subject is I (1st person singular), so you don’t need the pronoun:

(Minä) kuljen = I walk / I go

You use Minä mostly when you want to emphasize the subject:

Minä kuljen usein polkua pitkin metsään, mutta veljeni ei.
→ I often walk along the path into the forest, but my brother doesn’t.

In neutral sentences, dropping Minä is very natural.

Why is the verb kuljen and not kävelen or menen? What’s the difference?

All three can involve movement, but they feel different:

kulkea (kuljen)
- fairly neutral “to go / travel / move along”, often used for moving along a route or traffic
- can be by walking, driving, etc., depending on context
kävellä (kävelen)
- specifically “to walk (on foot)”
mennä (menen)
- very general “to go”

In this sentence:

Minä kuljen usein polkua pitkin metsään.

kuljen suggests you regularly travel / go along this path, often used about habitual routes.

If you say:

Minä kävelen usein polkua pitkin metsään.

the focus is more explicitly on walking on foot.
Both are fine; the nuance is slightly different.

Where can I put usein (“often”)? Is its position fixed?

Usein is quite flexible. Common options:

Minä kuljen usein polkua pitkin metsään.
Usein kuljen polkua pitkin metsään.
Minä usein kuljen polkua pitkin metsään.

All are grammatical and natural.
Slight nuances:

At the very beginning (Usein kuljen…) → mild emphasis on how often.
After the verb (kuljen usein…) → very neutral.
Between subject and verb (Minä usein kuljen…) → still fine, but can slightly highlight Minä or give a “storytelling” rhythm.

You would not normally split polkua pitkin in a strange way or stick usein inside that phrase.

Why doesn’t Finnish use articles here, like “the path” or “a forest”?

Finnish does not have articles (a, an, the) at all.
Whether English would say “a” or “the” is usually expressed in Finnish:

by context
by word order
by case endings
sometimes by additional words (like juuri se polku = “that exact path”).

So:

polkua can translate as “a path” or “the path” depending on context.
metsään can be “into a forest” or “into the forest”.

In your sentence, a natural translation chooses “the path” and “the forest” because it feels like a familiar, known route, but the Finnish grammar itself doesn’t mark that difference.

Is the word order Minä kuljen usein polkua pitkin metsään the only correct one?

No. Finnish word order is flexible, especially in written language, as long as the case endings stay correct. Some natural variants:

Kuljen usein polkua pitkin metsään.
Usein kuljen polkua pitkin metsään.
Minä kuljen polkua pitkin usein metsään. (possible, but the adverb feels slightly detached)
Minä kuljen pitkin polkua usein metsään. (also possible but stylistically less smooth)

What you cannot freely change:

polkua must stay in partitive with pitkin.
metsään must stay in illative to keep the meaning “into the forest”.

Finnish uses word order more for emphasis and information structure than for basic grammar.

Could I say Minä kuljen usein metsässä polkua pitkin instead?

You can, but the meaning changes slightly.

Minä kuljen usein polkua pitkin metsään.
→ You go along the path into the forest (movement towards/into).
Minä kuljen usein metsässä polkua pitkin.
→ You are already in the forest, and you move along a path there.
(No clear idea of “entering” the forest.)

So metsään (into) vs. metsässä (in) is important for the direction of movement.

Why does the verb look like kuljen instead of just kulke plus “I”?

In Finnish, person and number are marked on the verb itself, instead of using separate pronouns and auxiliary verbs.

The verb is kulkea (to go / to travel / to move).
Its present tense forms:

minä kuljen – I go
sinä kuljet – you go
hän kulkee – he/she goes
me kuljemme – we go
te kuljette – you (pl.) go
he kulkevat – they go

So the -n ending in kuljen tells you the subject is I, which is why you can omit Minä.

Is there any sound or spelling change in kulkea → kuljen that I should notice?

Yes, this is a common pattern with verbs like kulkea:

stem: kulke-
1st person singular: kuljen, not *kulkeen

Two things happen:

The -ea ending of the infinitive changes; the personal forms are built on a slightly different stem.
There is a consonant alternation k → j inside the stem:
- kulke- → kulje- → kuljen.

You’ll see similar patterns with other verbs ending in -kea / -keä. It’s worth memorizing as a typical verb-type behavior in Finnish.