Kuljen metrolla harvemmin kuin bussilla.

Kuljen is the 1st person singular present tense of kulkea.

In this sentence, kulkea means something like go / travel / get around. It often describes a general way of moving from place to place, especially as a habit or usual pattern.

So here it is not specifically walk. Because the sentence includes metrolla and bussilla, the meaning is clearly about travelling by those means of transport.

Finnish usually leaves out subject pronouns when they are obvious from the verb ending.

• kuljen = I go / I travel
• the ending -n already tells you the subject is I

So Kuljen metrolla... is perfectly normal.
You could say Minä kuljen metrolla..., but that would usually add emphasis or contrast, for example if you are comparing yourself with someone else.

They are in the adessive case, which has the ending -lla / -llä.

With means of transport, Finnish often uses this case to express by:

• metrolla = by metro
• bussilla = by bus
• junalla = by train
• autolla = by car

So instead of using a separate word like English by, Finnish often changes the noun ending.

Not exactly. The adessive case often has meanings like on, at, or with, depending on context.

With vehicles, it commonly expresses the means of transport, so English usually translates it as by:

• bussilla → by bus
• metrolla → by metro

So even though the same case can mean on in other contexts, here the most natural English translation is by.

Because the case ending itself is -lla, with a double l.

So the word is formed like this:

• metro + lla = metrolla
• bussi + lla = bussilla

The double l is not something special happening only to metro; it is just part of the normal ending.

Harvemmin is a comparative adverb.

Its basic form is harvoin, meaning rarely or seldom.
The comparative form harvemmin means less often or more rarely.

So:

• harvoin = rarely
• harvemmin = less often

In this sentence, it compares how often the speaker travels by metro versus by bus.

Kuin is the usual word for than after a comparative in Finnish.

So:

• harvemmin kuin = less often than
• useammin kuin = more often than
• nopeammin kuin = faster than

In this sentence, kuin connects the two things being compared:

• metrolla
• bussilla

Normally, no.

Even though kulkea can sometimes refer more generally to moving around, in this sentence the forms metrolla and bussilla strongly suggest means of transport. So the natural interpretation is about travelling by metro and travelling by bus, not about physically walking inside them.

No, Finnish word order is fairly flexible. The version given is natural and neutral:

• Kuljen metrolla harvemmin kuin bussilla.

But other orders are also possible, with slightly different emphasis:

• Metrolla kuljen harvemmin kuin bussilla.
  This puts more focus on metrolla.

• Kuljen harvemmin metrolla kuin bussilla.
  Also possible, with a slightly different rhythm or emphasis.

So the basic meaning stays the same, but word order can highlight different parts of the sentence.

All three can relate to movement, but they are not exactly the same.

• kulkea = go, travel, get around; often about a usual way of moving around
• mennä = go; often more about a specific act of going somewhere
• matkustaa = travel; often sounds broader, sometimes more like making a trip or journey

In this sentence, kulkea is very natural because it sounds like a general habit or regular pattern: the speaker is comparing how often they use different forms of transport.

Because Finnish has no articles like English a or the.

So metro and bussi do not need an extra word in front of them. Context tells you whether the meaning is general or specific.

In this sentence, the meaning is usually general:

• travelling by metro
• travelling by bus

rather than talking about one particular metro train or one particular bus.