Maasto on mutainen, joten kuljemme pitkospuita pitkin.

Maasto means terrain, ground, or the landscape as something you move through.

It is a good word when talking about hiking, walking conditions, routes, forests, marshes, and so on. It is more specific than maa, which can mean land, soil, ground, or country depending on context.

So Maasto on mutainen is very naturally The terrain is muddy.

Mutainen is an adjective meaning muddy.

It comes from:

• muta = mud
• -inen = a common adjective-forming ending

So:

• muta → mutainen = muddy

This -inen adjective type is very common in Finnish:

• tuuli → tuulinen = windy
• lumi → luminen = snowy
• kivi → kivinen = stony, rocky

In this sentence, mutainen is the basic singular form because it is a predicate adjective describing maasto.

Because the adjective is acting as a predicate adjective after on (is), and it normally agrees with the subject.

Here:

• maasto = singular nominative
• so the adjective is also in the singular nominative: mutainen

So:

• Maasto on mutainen = The terrain is muddy

This is the normal pattern:

• Tie on pitkä. = The road is long.
• Sää on kylmä. = The weather is cold.
• Maasto on mutainen. = The terrain is muddy.

Joten means so, therefore, or thus.

It introduces a result or consequence:

• Maasto on mutainen, joten...
• The terrain is muddy, so...

So the logic is:

1. the terrain is muddy
2. therefore we go along the duckboards

It is a very useful connector when one clause explains the consequence of the previous clause.

Because Finnish normally uses a comma to separate clauses.

Here you have two clauses:

• Maasto on mutainen
• joten kuljemme pitkospuita pitkin

So the comma is standard:

• Maasto on mutainen, joten kuljemme pitkospuita pitkin.

This is one difference from English punctuation habits: Finnish is generally quite consistent about separating clauses with commas.

Kuljemme is the 1st person plural present tense of kulkea.

So:

• kulkea = to go, to travel, to move along, to walk
• kuljemme = we go / we travel / we walk

The ending -mme is the usual we ending in Finnish verbs.

For example:

• kuljen = I go
• kuljet = you go
• kulkee = he/she/it goes
• kuljemme = we go
• kuljette = you (plural) go
• kulkevat = they go

Because kulkea is broader than kävellä.

• kävellä = to walk, focusing on the act of walking
• kulkea = to go, move, travel, proceed, often focusing on the route or manner of movement

In a sentence about choosing a route through muddy terrain, kulkea sounds very natural because the point is not just that we are walking, but that we are proceeding along a particular path.

So kuljemme pitkospuita pitkin is something like:

• we go along the duckboards
• we proceed along the duckboards
• we walk along the duckboards

Pitkospuut are duckboards or wooden planks laid across wet ground, especially on hiking trails in marshy or muddy areas.

This is a very Finnish word because these are common in nature trails and boggy terrain.

A useful thing to know is that this word is very often used in the plural, because the walkway consists of several planks:

• pitkospuut = duckboards / the plank walkway

The singular pitkospuu exists, but in real use the plural is extremely common when talking about the structure you walk on.

Because here the word is in the partitive plural:

• basic form: pitkospuut
• partitive plural: pitkospuita

This happens because of the construction with pitkin.

A very common Finnish pattern is:

• tietä pitkin = along the road
• jokea pitkin = along the river
• pitkospuita pitkin = along the duckboards

So pitkospuita is not the subject here. It is part of an expression meaning along X.

Pitkin means along.

In this sentence:

• pitkospuita pitkin = along the duckboards

This is a common route-expression in Finnish. It tells you the line or surface that someone moves along.

Compare:

• tietä pitkin = along the road
• rantaa pitkin = along the shore
• jokea pitkin = along the river
• pitkospuita pitkin = along the duckboards

So the whole sentence means that because the terrain is muddy, we move along the duckboards instead.

Yes, Finnish word order is fairly flexible, although the original sentence is the most neutral and natural version.

Neutral:

• Maasto on mutainen, joten kuljemme pitkospuita pitkin.

You could change the order for emphasis, for example:

• Maasto on mutainen, joten pitkospuita pitkin kuljemme.

That version emphasizes pitkospuita pitkin, but it sounds more marked or literary. A learner should usually stick with the original order unless there is a special reason to highlight something.

So the given sentence is a very good standard model.