Learn Hardcore Finnish: Aamulla sumu on paksu ja lämpötila on matala, joten meidän pitää ajaa hitaasti. - In the morning the fog is thick and the temperature is low, so we have to drive slowly.

So Aamulla sumu on paksu… literally means “In the morning the fog is thick…”. Using just aamu without a case ending would be ungrammatical here.

Could I also say aamuna or aamussa? What’s the difference from aamulla?

All three forms exist, but they’re used differently:

aamulla (adessive) – the normal, default way to say “in the morning” in everyday speech.
aamuna (essive) – means “as a morning / on a (certain) morning”, often for one specific morning or in slightly more literary style:
- Eräänä aamuna sumu oli hyvin paksu. – On one morning the fog was very thick.
aamussa (inessive) – theoretically “in the morning”, but this is rare and marked. You might see it only in set expressions or poetic language.

In your sentence, the natural choice is aamulla.

Can I change the word order? For example, can I say Sumu on paksu aamulla?

Yes, both are correct:

Aamulla sumu on paksu…
Sumu on paksu aamulla…

The difference is focus:

Aamulla sumu on paksu… – starts with when; emphasizes the time (“In the morning, the fog is thick…”).
Sumu on paksu aamulla… – starts with what; emphasizes the fog, then adds when as extra information.

Finnish word order is fairly flexible, but the version with Aamulla first is very natural here because time expressions often appear at the beginning.

In sumu on paksu, what form is paksu in, and why is it not paksua?

Paksu is in the nominative case, used here as a predicative adjective:

sumu (subject, nominative)
on (verb olla, “to be”)
paksu (predicative, nominative)

You might have heard that predicatives can also be in partitive (e.g. paksua). Rough guideline:

nominative predicative (paksu) – when the state/quality is seen as complete, definite, or characteristic.
- Sumu on paksu. – The fog is (clearly) thick.
partitive predicative (paksua) – more indefinite, partial, or in progress, or when you’re saying “there is …” rather than describing a specific subject:
- On paksua sumua. – There is thick fog. / There’s a lot of thick fog.

In your sentence, we are describing the fog as having a clear property, so paksu is used.

Why say lämpötila on matala instead of just on kylmä?

Both are possible but they are slightly different ideas:

On kylmä. – “It is cold.” (general feeling; no explicit mention of temperature)
Lämpötila on matala. – “The temperature is low.” (talks specifically about the temperature value, more neutral/technical)

In this sentence, lämpötila on matala matches well with sumu on paksu: both parts describe measurable or observable conditions (fog thickness and temperature level) that justify driving slowly.

What exactly does joten mean, and how is it different from koska or siksi?

Joten is a conjunction meaning roughly “so / therefore / and so”. It introduces a result or consequence:

…sumu on paksu ja lämpötila on matala, joten meidän pitää ajaa hitaasti.
“…the fog is thick and the temperature is low, so we have to drive slowly.”

Compare:

koska – means “because” and introduces the reason:
- Meidän pitää ajaa hitaasti, koska sumu on paksu…
  We have to drive slowly because the fog is thick…
siksi – means “for that reason / that’s why”, and is usually used with että or at the start of a new clause:
- Sumu on paksu, siksi meidän pitää ajaa hitaasti.
- Sumu on paksu, ja siksi meidän pitää ajaa hitaasti.

So:

Use joten between clauses when you want to say “…, so …”.
Use koska to say “because …”.
Use siksi more like “that’s why”.

Why is there a comma before joten?

In Finnish, independent clauses joined by conjunctions like joten, mutta, vaan, sillä are usually separated by a comma.

Your sentence has two clauses:

Aamulla sumu on paksu ja lämpötila on matala
meidän pitää ajaa hitaasti

They are joined by joten, so a comma is required:

…matala, joten meidän pitää ajaa hitaasti.

(Inside the first clause, the small ja connecting sumu on paksu and lämpötila on matala does not take a comma, because that’s just a standard “X and Y” structure within a single clause.)

What is the structure meidän pitää? Why meidän and not me?

Meidän pitää literally means “(it) must for us”, and together it is best translated “we have to / we must”.

me = we (basic pronoun form, nominative)
meidän = our / of us (genitive)

With pitää in the sense of “must / have to”, the logical subject (the one who has the obligation) is put in the genitive:

Minun pitää… – I must / I have to…
Sinun pitää… – You must / you have to…
Hänen pitää… – He/she must…
Meidän pitää… – We must…
Teidän pitää… – You (pl) must…
Heidän pitää… – They must…

So meidän is in the genitive because that’s how this necessity construction works. Using just me pitää would be wrong.

How does pitää work with another verb like ajaa? Why is it not meidän ajamme hitaasti?

When pitää expresses necessity (“must / have to”), it is followed by the 1st infinitive (dictionary form) of the main action:

meidän pitää ajaa – we must drive
meidän pitää mennä – we must go
meidän pitää odottaa – we must wait

So the structure is:

[genitive pronoun] + pitää + [verb in basic form]

If you say me ajamme hitaasti, that simply means “we drive slowly”, with no idea of obligation. Your sentence wants to say “we have to drive slowly”, so we use meidän pitää ajaa hitaasti.

What is hitaasti, and how is it related to hidas?

Hitaasti is an adverb meaning “slowly”.

It is formed from the adjective hidas (“slow”) by adding the adverb ending -sti:

hidas – slow → hitaasti – slowly
nopea – fast → nopeasti – quickly
hiljainen – quiet → hiljaa / hiljaisesti – quietly (common form: hiljaa)

So:

hidas auto – a slow car
Auto ajaa hitaasti. – The car drives slowly.

In your sentence, hitaasti modifies ajaa (“drive”), so it’s in adverb form.

Can I leave out meidän and just say pitää ajaa hitaasti?

Yes, you can, but the meaning changes slightly.

Meidän pitää ajaa hitaasti. – We have to drive slowly.
Pitää ajaa hitaasti. – (One) has to drive slowly / Driving slowly is necessary.

Without meidän, the sentence becomes impersonal or generic. It can mean:

“People must drive slowly.”
“You (in general) have to drive slowly here.”

If the speaker specifically means “we” (the speaker’s group), it’s clearer to keep meidän.

Why are the verbs on and pitää in the present tense when we’re talking about the morning (possibly in the future)?

Finnish usually uses the present tense to talk about future events, especially when there is a time expression making the time clear:

Huomenna sataa. – It will rain tomorrow.
Illalla menen kotiin. – I will go home in the evening.

In your sentence:

Aamulla sumu on paksu…
…meidän pitää ajaa hitaasti.

The time adverb aamulla already tells us this is about (that) morning, which may be in the future or a general rule. Finnish doesn’t need a separate future tense; the present does the job when context/time words make the meaning clear.