Learn Hardcore Finnish: Professori oli jo lopettanut luennon, kun tulin luokkaan myöhässä. - The professor had already finished the lecture when I came to the classroom late.

Professori oli jo lopettanut luennon = The professor had already finished the lecture (before something else in the past happened).
kun tulin luokkaan myöhässä = when I came into the classroom late.

So the sentence contrasts two past events:

The professor finished the lecture (earlier past).
You came in late (later past).

Finnish often shows this relationship by using:

pluperfect for the earlier past action (oli lopettanut),
simple past for the later past action (tulin).

You could say Professori lopetti luennon, kun tulin luokkaan myöhässä, and it is grammatically fine, but it sounds more like both events form one simple sequence, without highlighting that the lecture was already over when you arrived. The pluperfect makes that clearer and more natural here.

What does jo mean here, and what nuance does it add?

Jo means “already”.

Without jo: Professori oli lopettanut luennon – The professor had finished the lecture.
With jo: Professori oli jo lopettanut luennon – The professor had already finished the lecture.

Jo suggests that:

the action is completed sooner than expected or
the timing is relevant/surprising from the speaker’s point of view.

Typical alternatives:

juuri = just / just now → Professori oli juuri lopettanut luennon (had just finished).
vihdoin = finally → Professori oli vihdoin lopettanut luennon (had finally finished).

The position of jo is usually before the verb phrase: oli jo lopettanut, but you may also see it before the participle in some cases (especially spoken language).

Why is it luennon and not luentoa or luento?

Luento = lecture (basic dictionary form, nominative).

Luennon is the genitive singular form of luento and here it is used as a total object of lopettaa (“to finish, to stop [something]”).

In Finnish, the object case changes depending on:

whether the action is seen as complete/whole (total object → usually genitive)
or incomplete/ongoing/partial (partitive object).

Here, the professor finished the whole lecture, so the lecture is a total object:

Professori lopetti / oli lopettanut luennon.
The professor (had) finished the lecture (completely).

If the action were incomplete or repeated in a different context, you might see the partitive:

Professori kuunteli luentoa. – The professor was listening to (some of) the lecture.
Professori lopetti luentoa is odd, because you can’t really “partially finish” a lecture in this sense.

So luennon is the correct total object form here.

Why is it luokkaan and not luokassa or luokkaan sisälle?

Finnish has different location cases. For luokka (classroom), the key ones are:

luokassa – in the classroom (inessive)
luokkaan – into the classroom (illative, direction towards the inside)
luokasta – out of the classroom (elative)

In tulin luokkaan, you are talking about movement into the classroom, so the illative -an is used:

tulin luokkaan = I came into the classroom.

If the sentence described a static location, you’d use luokassa:

Olin luokassa = I was in the classroom.

Luokkaan sisälle would be redundant; luokkaan already includes the idea of going in. You might still hear it in colloquial speech for emphasis, but tulin luokkaan is perfectly natural and complete.

What is myöhässä exactly, and how is it different from myöhään or myöhemmin?

Myöhässä literally means “late” (as a state) and functions like a predicate or an adverb meaning late, delayed.

Common patterns:

Olen myöhässä. – I am late.
Tulin myöhässä. – I arrived late. (exactly as in the sentence)

Compare:

myöhässä – late (state of being late, delay in schedule)
- Bussi on myöhässä. – The bus is late.
myöhään – (until) late in the day / at a late hour
- Valvoin myöhään. – I stayed up late.
myöhemmin – later (at a later time than some reference point)
- Tulen myöhemmin. – I’ll come later.

In tulin luokkaan myöhässä, myöhässä tells us that your arrival time was later than it should have been, not just that it was at a late hour of the day.

Why is there no minä before tulin?

Finnish usually omits subject pronouns because the verb ending already shows the person and number.

tulin = I came (1st person singular)
tulit = you came
tuli = he/she/it came, etc.

So:

(Minä) tulin luokkaan myöhässä. – I came into the classroom late.

Adding minä is grammatically correct, but it adds emphasis or contrast, like:

Minä tulin luokkaan myöhässä, en sinä. – I was the one who came late, not you.

In a neutral narrative sentence like this, omitting minä is more natural.

What does kun mean here, and how is it different from koska?

In this sentence kun means “when” and introduces a time clause:

kun tulin luokkaan myöhässä – when I came into the classroom late.

So the structure is:

main clause: Professori oli jo lopettanut luennon
time clause with kun: kun tulin luokkaan myöhässä

Kun has two common uses:

when (time):
- Kun tulin kotiin, söin. – When I came home, I ate.
because (reason, more colloquial than koska):
- Kun olin väsynyt, menin nukkumaan. – Because I was tired, I went to sleep.

Koska is the more “pure” because and is not normally used just for “when”:

Koska olin myöhässä, professori oli jo lopettanut luennon. – Because I was late, the professor had already finished the lecture.

In your sentence, kun is clearly temporal: it marks the moment in the past when you came in.

Can I change the word order, for example start with Kun tulin…? Does it change the meaning?

Yes, you can change the word order quite freely without changing the basic meaning.

All of these are grammatically fine and mean essentially the same:

Professori oli jo lopettanut luennon, kun tulin luokkaan myöhässä.
Kun tulin luokkaan myöhässä, professori oli jo lopettanut luennon.

The difference is mainly information structure / emphasis:

Starting with Professori oli… focuses first on the situation (lecture already finished), then explains when this was so.
Starting with Kun tulin… focuses first on your arrival, then states what the situation was at that time.

Within the main clause, you can also move jo a bit:

Professori oli jo lopettanut luennon.
Professori oli lopettanut jo luennon. (still okay, but more marked/colloquial)

But you cannot split fixed forms wrongly, for example *Professori oli lopettanut luennon jo oli would be incorrect.

How do I say “a professor” vs “the professor” in Finnish if there are no articles?

Finnish has no articles like a/an or the, so professori can mean “a professor” or “the professor” depending on context.

Professori oli jo lopettanut luennon.
- in a story where you know which professor → the professor
- in an out-of-the-blue mention → a professor

If you really want to emphasize indefinite (“a certain, one”) you can use eräs:

Eräs professori oli jo lopettanut luennon. – A (certain) professor had already finished the lecture.

To emphasize definiteness or “that specific one”, you can use se:

Se professori oli jo lopettanut luennon. – That (particular) professor had already finished the lecture.

As for capitalization: Professori is capitalized here only because it is the first word of the sentence. Profession names in Finnish are not normally capitalized otherwise.

What is the difference between lopettaa and loppua here?

lopettaa = to stop / finish something (transitive verb)
loppua = to end / be finished (intransitive verb)

So:

Professori lopetti luennon. – The professor finished (ended) the lecture.
(professori does the ending, luento is the thing being ended.)
Luento loppui. – The lecture ended.
(The lecture ends by itself; no explicit agent.)

In your sentence:

Professori oli jo lopettanut luennon uses lopettaa with an object (luennon), showing that the professor is the agent who actively ended the lecture.

You couldn’t say *Professori oli jo loppunut luennon – that’s ungrammatical because loppua doesn’t take an object.

How is oli lopettanut (the pluperfect) formed in Finnish?

The pluperfect is formed with:

the past tense of olla (to be): olin, olit, oli, olimme, olitte, olivat
the past active participle of the main verb: typically -nut/-nyt or -nut/-nne variations.

For lopettaa:

past participle: lopettanut (sg), lopettaneet (pl)

So:

olin lopettanut – I had finished
olimme lopettaneet – we had finished
oli lopettanut – he/she had finished

In your sentence:

Professori oli jo lopettanut luennon
- oli = past of olla (3rd person singular)
- lopettanut = past active participle of lopettaa

Together: oli lopettanut = had finished.

Why does luento become luennon? What’s happening with the nn?

This is a combination of:

Case change (nominative → genitive), and
consonant gradation / stem change.

Dictionary form: luento (lecture)
Stem used in many cases: luenn-

So:

nominative: luento – lecture
genitive: luennon – of the lecture / (here) the lecture as total object
partitive: luentoa
illative plural: luentoihin, etc.

The nt → nn change is a regular pattern in many -nto nouns:

tieto → tiedon, but
kysyntä → kysynnän
tulento (hypothetical) → tulennon

You just have to learn that luento has the stem luenn-, so luennon is the correct genitive form used for the total object in your sentence.

Could I say Professori oli juuri lopettanut luennon instead of jo? What’s the difference?

Yes, you can, but the nuance changes:

jo = already
→ Professori oli jo lopettanut luennon:
The professor had already finished the lecture (earlier than your arrival; maybe earlier than expected).
juuri = just / just now
→ Professori oli juuri lopettanut luennon:
The professor had just finished the lecture (very shortly before you came in).

So jo emphasizes that the finishing was done and in the past relative to you;
juuri emphasizes that it happened very recently before your arrival.