Learn Hardcore Finnish: Luento oli pitkä, mutta professori selitti vaikean aiheen selvästi. - The lecture was long, but the professor explained the difficult topic clearly.

Luento oli pitkä can mean “The lecture was long” or “A lecture was long,” depending on context.
professori likewise can mean “the professor” or “a professor.”

Definiteness in Finnish is usually understood from context, word order, or extra words if needed (se professori = “that/the professor,” eräs professori = “a certain professor”). The bare noun is the normal way to say both “a X” and “the X.”

What exactly does oli mean here, and what tense is it?

oli is the 3rd person singular past tense of the verb olla (“to be”).

olla = to be
oli = he/she/it was

So Luento oli pitkä literally is “Lecture was long”, understood in English as “The lecture was long.” Finnish uses this simple past form for what English expresses as “was,” and there is no separate “past continuous” form like “was being long.”

Why is pitkä in its basic form, but vaikean and aiheen have -n at the end?

This difference comes from case and sentence role.

In Luento oli pitkä, pitkä is a predicative adjective (describing the subject via “to be”). Predicative adjectives normally appear in the nominative (basic) form when the subject is singular:
- luento (lecture, nominative)
- pitkä (long, nominative)
In vaikean aiheen, both words end in -n. That is the genitive singular ending. Here:
- aiheen = “of the topic / the topic” in genitive
- vaikean agrees with aiheen in case, so it also takes -n.

So:

pitkä is nominative because it is a predicative after oli.
vaikean aiheen is genitive because it functions as a certain kind of object (a total object, explained next).

What case is vaikean aiheen, and why is the object not in the partitive like vaikeaa aihetta?

vaikean aiheen is in the genitive singular. This is the standard form for a total (complete) object in many active past-tense sentences.

professori selitti vaikean aiheen selvästi
= The professor explained the entire difficult topic clearly (the task is seen as completed).

If you used the partitive instead:

professori selitti vaikeaa aihetta selvästi

then the meaning shifts. The partitive object often suggests:

an ongoing process or incomplete action, or
focusing on the activity itself, not its completion.

So vaikean aiheen implies that the professor managed to explain the whole difficult topic; vaikeaa aihetta would sound more like “was (in the process of) explaining a difficult topic.”

Why do both vaikean and aiheen have the same ending? Do adjectives always follow the noun’s case?

Yes, when an adjective directly modifies a noun (an attributive adjective), it normally agrees with that noun in case and number.

Nominative (basic form):
- vaikea aihe = a difficult topic
Genitive:
- vaikean aiheen = of the difficult topic / the difficult topic (as total object)
Partitive:
- vaikeaa aihetta = (some) difficult topic / a difficult topic (as partitive object)

In the sentence, aiheen is genitive, so vaikea must also become vaikean to match it. This is standard agreement between adjectives and the nouns they modify.

Could I say professori selitti vaikeaa aihetta selvästi instead? How would the meaning change?

You can, but it changes the nuance:

professori selitti vaikean aiheen selvästi
→ He explained the whole difficult topic clearly (completed, result-focused).
professori selitti vaikeaa aihetta selvästi
→ He was explaining a difficult topic clearly (process-focused, not necessarily finished, or we don’t care whether he covered the whole topic).

The difference is subtle but important:

Genitive object: more bounded / complete.
Partitive object: more unbounded / ongoing / partial or sometimes repeated/habitual.

What is selvästi, and how is it formed from selvä?

selvästi is an adverb meaning “clearly.” It is formed from the adjective selvä (“clear”) by adding the adverbial ending -sti to the adjective stem:

selvä (clear) → selvästi (clearly)
Similarly:
- nopea (fast) → nopeasti (quickly)
- hidas (slow) → hitaasti (slowly)
- hyvä (good) → hyvin (well; this one is irregular, not hyvästi in this meaning)

So selvästi answers “how?” → How did he explain? Clearly.

Does selvästi have to go at the end, or can its position change?

Adverbs like selvästi are fairly flexible in Finnish word order. All of these are grammatical, with only slight differences in emphasis:

Professori selitti vaikean aiheen selvästi.
Professori selitti selvästi vaikean aiheen.
Professori selitti vaikean aiheen hyvin selvästi.
Selvästi professori selitti vaikean aiheen. (strong emphasis on “clearly”)

The version in the original sentence (...aiheen selvästi) is very natural and neutral. Moving selvästi earlier often puts more emphasis on how he explained, but the basic meaning stays the same.

What does mutta do to the sentence? Does it change the word order after it?

mutta is a coordinating conjunction meaning “but.” It does not cause any special word order change like inversion.

Before mutta: Luento oli pitkä (Subject–Verb–Complement)
After mutta: professori selitti vaikean aiheen selvästi (Subject–Verb–Object–Adverb)

The word order after mutta stays the normal SVO type. mutta simply links two clauses that contrast with each other:

Long lecture but clear explanation.

Why is there a comma before mutta?

In Finnish, a comma is used to separate independent clauses joined by conjunctions like mutta (“but”), ja (“and”) in many cases, tai (“or”), etc.

Here we have two full clauses:

Luento oli pitkä
professori selitti vaikean aiheen selvästi

Each has its own subject and verb, so they are separated by a comma:

Luento oli pitkä, mutta professori selitti vaikean aiheen selvästi.

This is similar to English punctuation, although English is a bit more flexible about commas with and/but.

Is the word order professori selitti vaikean aiheen selvästi fixed, or could I start with vaikean aiheen?

You can move elements around, especially for emphasis or information structure. For example:

Professori selitti vaikean aiheen selvästi.
→ neutral: “The professor explained the difficult topic clearly.”
Vaikean aiheen professori selitti selvästi.
→ focus on the topic: “It was the difficult topic that the professor explained clearly.”
(Sounds somewhat marked/emphatic, but correct.)
Selvästi professori selitti vaikean aiheen.
→ strong focus on “clearly.”

Finnish allows relatively free word order because case endings show the roles of words. The original order (Subject–Verb–Object–Adverb) is the most neutral, “default” style.