Learn Hardcore Finnish: Jos ruuhka on paha, menen keskustaan metrolla. - If the traffic is bad, I go to the city center by metro.

Jos means if and introduces a conditional clause. In Finnish, when an if-clause comes first, it’s typically followed by a comma, and the main clause comes after it:
Jos X, (niin) Y.
Here: Jos ruuhka on paha, menen... = If the congestion is bad, I go/I’ll go...
You can also add niin (then) in the main clause for emphasis: Jos ruuhka on paha, niin menen... (often optional).

Why is there a comma in Jos ruuhka on paha, menen...?

Finnish normally uses a comma between a subordinate clause and the main clause when the subordinate clause comes first. The jos-clause is subordinate, so you write:
Jos ruuhka on paha, menen...
If you reverse the order, the comma is usually still used: Menen keskustaan metrolla, jos ruuhka on paha.

What exactly does ruuhka mean here?

Ruuhka means congestion (traffic congestion, crowdedness, rush-hour congestion). In everyday speech it often implies traffic (especially in commuting contexts), but it can also refer to crowds in general depending on context.

Why is it ruuhka on paha and not something like ruuhka on pahaa?

Because paha is functioning as a predicate adjective describing the subject ruuhka. Predicate adjectives agree with the subject in number and typically appear in the basic form (nominative singular here):

Ruuhka on paha = The congestion is bad.
Using pahaa would be partitive and would usually suggest an uncountable “some badness” or a different structure; it’s not the normal way to say “is bad” about ruuhka.

Does paha always mean “bad,” or can it be translated differently?

Paha most commonly means bad (negative, unpleasant, severe). With ruuhka, it can also feel like heavy, severe, or really bad depending on context. Finnish has other common choices too, like kova (heavy/intense) or kamala (awful), but paha is perfectly natural.

Why is menen in the present tense if the meaning is about the future (like “I’ll go”)?

Finnish often uses the present tense to talk about future actions when the time is clear from context (here, it’s conditional and clearly about what you will do in that situation).
So menen can mean I go or I will go depending on context. If you want to emphasize intention, you can also use aion mennä (I intend to go).

Why isn’t minä (I) included? Is it optional?

Yes, it’s usually optional because Finnish verb endings show the person. Menen already means I go / I’ll go.
You’d include minä mainly for emphasis or contrast:

Jos ruuhka on paha, minä menen keskustaan metrolla (emphasizing “I” specifically)

What case is keskustaan, and what does that ending mean?

Keskustaan is the illative case of keskusta (city center/downtown). The illative expresses movement into/to a place (destination).

mennä keskustaan = to go to downtown / to the city center
The form is keskusta → keskustaan (with the long vowel -aan typical for many illatives).

How is keskustaan different from keskustassa or keskustasta?

They’re different location-direction cases:

keskustaan (illative) = to/into downtown (destination)
keskustassa (inessive) = in downtown (location)
keskustasta (elative) = out of/from downtown (origin)

Why is it metrolla? What does the -lla ending mean?

Metrolla is metro + -lla (adessive case). In this context, adessive expresses means of transport:

metrolla = by metro / on the metro
Other examples: autolla (by car), bussilla (by bus), pyörällä (by bike).

Could I also say metroon or metrillä instead of metrolla?

metrolla is the standard way to say by metro (means of transport).
metroon (illative) means into the metro / onto the metro (entering it), focusing on boarding: Menen metroon = “I go into the metro (system/train).”
metrillä is not standard; the correct adessive is metrolla.

Is the word order fixed, or could parts be rearranged?

Finnish word order is flexible, but changes emphasis. The neutral version is exactly what you have:
Jos ruuhka on paha, menen keskustaan metrolla.
You can move elements for focus:

Jos ruuhka on paha, metrolla menen keskustaan. (emphasizes by metro)
Jos ruuhka on paha, keskustaan menen metrolla. (emphasizes to downtown)
The meaning remains similar, but the “spotlight” shifts.