Learn Hardcore Finnish: Bussi kulkee hitaasti ruuhkassa. - The bus moves slowly in the traffic jam.

Finnish has no articles like “a/an” or “the”.
Bussi can mean “a bus” or “the bus” depending on context.
So Bussi kulkee hitaasti ruuhkassa can be translated as either “The bus goes slowly in the traffic jam” or “A bus goes slowly in the traffic jam,” depending on what you are talking about in the wider context.

Why is bussi capitalized? Are Finnish nouns always written with a capital letter?

No, Finnish nouns are not capitalized in general.
Bussi is capitalized here only because it is the first word of the sentence.
If it were in the middle of a sentence, it would be written bussi, with a lowercase b.

What exactly does kulkee mean, and how is it different from menee or ajaa?

Kulkea means “to move, to travel, to go (along a route)” and is often used for vehicles, public transport, or things that regularly operate along a path (e.g. juna kulkee – “the train runs/goes”).

menee (from mennä) is more general: “goes.”
ajaa means “to drive” or “to ride (in a vehicle).”

In this sentence, kulkee suggests the bus is moving / operating along its route slowly in the traffic jam, which sounds more natural than menee or ajaa for a scheduled bus.

What form of the verb is kulkee, and what is the basic dictionary form?

The dictionary form (infinitive) is kulkea.
Kulkee is the 3rd person singular present tense form:

minä kuljen – I go / travel
sinä kuljet – you go / travel
hän kulkee – he/she/it goes / travels

So Bussi kulkee literally means “The bus goes/travels.”

How is the adverb hitaasti formed from hidas?

The adjective is hidas = “slow.”
Finnish often forms adverbs of manner from adjectives with the ending -sti.
Here, hidas → hitaasti (note the vowel change: a appears twice in the stem: hita- + -asti).
So hitaasti = “slowly.”
This is similar to English “slow” → “slowly.”

Is hitaasti always placed before ruuhkassa, or can the word order change?

The neutral word order here is Bussi kulkee hitaasti ruuhkassa = Subject–Verb–Adverb–Location.
You can say Bussi kulkee ruuhkassa hitaasti, and it is still correct; the focus shifts slightly toward “in the traffic jam”.
Finnish word order is relatively flexible, but Bussi kulkee hitaasti ruuhkassa is a very natural, unmarked way to say it.

What case is ruuhkassa, and what does it literally mean?

Ruuhkassa is in the inessive case, which has the ending -ssa / -ssä and usually means “in, inside”.
The noun is ruuhka = “traffic jam,” “congestion,” or “rush (hour) traffic.”
So ruuhkassa literally means “in (a/the) traffic jam” or “in congestion.”

Why do we use the inessive -ssa (ruuhkassa) and not some other case?

The inessive -ssa/-ssä is used for being inside something or within a situation.
Heavy traffic or a traffic jam is treated like an environment that you are “in,” so ruuhkassa = “in (the) traffic jam.”
Other cases would change the meaning; for example, ruuhkaan (illative) would mean “into the traffic jam,” focusing on movement into it rather than being in it.

Can ruuhkassa also mean “in rush hour” or “in heavy traffic,” not just “in a literal standstill traffic jam”?

Yes. Ruuhka can mean:

a literal traffic jam / congestion, or
more generally, rush hour / very heavy traffic.

So ruuhkassa can be understood as “in (the) rush-hour traffic”, “in heavy traffic,” or “in a traffic jam,” depending on context.

Is bussi the subject, and is it in some special form?

Yes, bussi is the subject of the sentence.
It is in the nominative singular form, which is the basic, dictionary-like form used for subjects: bussi = “bus.”
So grammatically the structure is:

Subject: Bussi
Verb: kulkee
Adverb: hitaasti
Location adverbial (inessive): ruuhkassa

How would I say “The bus is going slowly in the traffic jam” versus “The bus goes slowly in the traffic jam”? Does Finnish distinguish these?

Finnish typically uses the same present tense for both simple and progressive meanings.
So Bussi kulkee hitaasti ruuhkassa can mean both:

“The bus goes / travels slowly in the traffic jam” (general description), and
“The bus is going / is travelling slowly in the traffic jam” (right now).

Context decides whether you interpret it as a general statement or something happening at this moment.

Could I say “Bussi on hitaasti ruuhkassa” to mean “The bus is slowly in the traffic jam”?

No, that would be ungrammatical or at least very odd.
The verb olla (“to be”) is not used with an adverb of manner (hitaasti) like this in Finnish.
To express that the bus is moving slowly, you need a motion verb such as kulkea, mennä, or ajaa; so Bussi kulkee hitaasti ruuhkassa is the natural way to say it.

How do you pronounce kulkee and ruuhkassa, especially the double letters?

kulkee: KUL-kee. Stress on the first syllable KUL. The double ee is a long e sound, held slightly longer than a single e.
ruuhkassa: RUUH-kas-sa. Stress on RUUH.
- uu is a long u sound.
- kk and ss are geminate consonants: you hold the consonant a bit longer, like a tiny pause before releasing it.

In Finnish, double vowels and consonants are meaningful; length can change the word’s meaning, so it’s worth practicing.