Learn Hardcore Finnish: Opettaja selittää asian lyhyesti. - The teacher explains the matter briefly.

Opettaja selittää lyhyesti asian. – Slight emphasis shift to asian at the end.
Asian selittää opettaja lyhyesti. – Emphasis on asian (the thing being explained).
Lyhyesti opettaja selittää asian. – Strong emphasis on how it is explained (briefly).

All of these are grammatically possible, but the original is the most neutral and typical.

Why is it selittää and not some other verb meaning explain, like selostaa or selostaa asian lyhyesti?

Several Finnish verbs can be translated as explain, but they have slightly different nuances:

selittää – to explain, clarify, make something understandable. Very common and neutral.
selostaa – to give an account, report, commentate, or describe in detail (e.g., a sports commentator selostaa the match).
perustella – to justify, give reasons.
valaista (figurative) – to illuminate, shed light on.

In this sentence, selittää is used because the teacher is just explaining something in a straightforward way. Selostaa would sound more like doing a detailed commentary or report, which does not fit the idea of doing it lyhyesti as well as selittää does.

How is the verb selittää formed and conjugated here? Why does it end in -ää?

Selittää is the dictionary (infinitive) form of the verb. It’s a type 1 verb in Finnish (ending in -a/ä in the infinitive).

The present tense conjugation (indicative) looks like this:

minä selitän – I explain
sinä selität – you explain
hän selittää – he/she explains
me selitämme – we explain
te selitätte – you (plural/formal) explain
he selittävät – they explain

In the sentence Opettaja selittää asian lyhyesti, the subject opettaja is third-person singular, so the verb takes the hän form: selittää.

The -ää ending is part of that 3rd person singular present form for type 1 verbs with this stem: selitä- → selittää.

Why is it asian and not asia or asiaa? What case is asian?

Asian is the genitive form of asia (matter, issue, thing).

In this sentence, asian is functioning as a total object of the verb selittää:

asia (nominative) – dictionary form
asian (genitive) – here: total object
asiaa (partitive) – would be a partial object

Finnish objects often appear either in:

Genitive (asian) when the action affects the object as a complete whole or is seen as bounded/total.
Partitive (asiaa) when the action is incomplete, unbounded, or only partially affecting the object.

Opettaja selittää asian lyhyesti implies that the teacher explains the whole matter (at least in some complete sense), just briefly.

You could hear Opettaja selittää asiaa in other contexts, but that would feel more like:

The teacher is explaining (about) the matter, with the focus on the ongoing process rather than a complete explanation.

Here, asian is the most natural choice.

Does asian mean specifically the matter (definite) or could it also mean a matter (indefinite)? How does Finnish show this?

Finnish does not have articles like English the or a/an, so asian can be translated as either:

the matter / the issue, or
a matter / an issue,

depending on context.

Definiteness (whether it’s the or a) is usually understood from:

Context (has this thing already been mentioned?)
Possessive constructions
Word order and emphasis

In isolation, Opettaja selittää asian lyhyesti is most naturally understood as the teacher explains the matter briefly, because asian sounds like something specific that is already known or is identifiable in the context.

Could you use asiaa instead of asian, and how would the meaning change?

Yes, you could say:

Opettaja selittää asiaa lyhyesti.

The difference:

selittää asian (genitive object) – implies a complete explanation of that matter (though done briefly).
selittää asiaa (partitive object) – implies explaining about the matter, perhaps only partially, or that the activity of explaining is in focus, not completion.

So:

Opettaja selittää asian lyhyesti. – The teacher gives a brief explanation of the matter (a whole, compressed explanation).
Opettaja selittää asiaa lyhyesti. – The teacher is explaining about the matter briefly; the completeness of the explanation is not emphasized.

Both are grammatically correct, but the original favors the idea of a short but complete explanation.

How is lyhyesti formed from lyhyt, and what does the ending -sti do?

Lyhyt is an adjective meaning short.
Lyhyesti is an adverb meaning briefly / in a short way.

The ending -sti is a common way to form adverbs from adjectives in Finnish, roughly like English -ly:

nopea (fast) → nopeasti (quickly)
hidas (slow) → hitaasti (slowly)
selvä (clear) → selvästi (clearly)
lyhyt (short) → lyhyesti (briefly)

Notice the stem change:

lyhyt → stem lyhye- → lyhyesti

That extra y comes from how the adjective stem is formed; many -yt adjectives use -ye- in derivatives:

tiivis → tiiviisti
lyhyt → lyhyesti

The important point: lyhyesti is the standard adverb form; lyhytsti would be incorrect.

Where can the adverb lyhyesti go in the sentence, and does changing its position change the meaning?

Adverbs like lyhyesti are quite flexible in Finnish word order. Some common positions:

Opettaja selittää asian lyhyesti.
– Neutral; adverb at the end is very typical.
Opettaja selittää lyhyesti asian.
– Slightly more emphasis on asian, since it comes last.
Lyhyesti opettaja selittää asian.
– Strong emphasis on lyhyesti; sounds more like Briefly, the teacher explains the matter.

All are grammatical. Moving lyhyesti forward tends to increase its prominence; keeping it at the end is the most neutral.

What tense is selittää here, and does it cover both is explaining and explains like in English?

Selittää in this sentence is in the present indicative.

Finnish present tense usually covers both:

English simple present (explains) and
English present continuous (is explaining)

So Opettaja selittää asian lyhyesti can mean:

The teacher explains the matter briefly. (habitual/general)
The teacher is explaining the matter briefly. (right now / current action)

Context decides which reading is more natural.

For the past, you would use:

Opettaja selitti asian lyhyesti. – The teacher explained the matter briefly.

How should selittää and lyhyesti be pronounced? Anything important about long vowels or consonants?

Key points:

selittää: se-lit-tää
- Stress on the first syllable: SE-lit-tää.
- tt is a long consonant; hold it longer than a single t.
- ää is a long vowel; pronounced like a long open front vowel (like a in English cat, but longer).
lyhyesti: ly-hy-es-ti (in practice often [ly.hy.es.ti])
- Again, stress on the first syllable: LY-hy-es-ti.
- y is a front rounded vowel, like the French u in lune or German ü in müde.
- No doubled consonants or vowels here, so everything is short.

Length differences in Finnish (single vs double consonants/vowels) are meaningful, so selittää and selitää would not be the same word.

Can the object asian be omitted, and what would Opettaja selittää lyhyesti mean?

Yes, the object can be omitted if it is clear from context or not important to mention.

Opettaja selittää lyhyesti.
→ The teacher explains briefly.

This leaves what is being explained unspecified. It could be understood from the larger conversation or situation. Grammatically it is fine; it simply becomes less specific.

Why is the subject opettaja instead of a pronoun like hän? Could you say Hän selittää asian lyhyesti?

You can absolutely say:

Hän selittää asian lyhyesti. – He/She explains the matter briefly.

Differences:

Opettaja selittää asian lyhyesti.
– Identifies the person by role: the teacher.
Hän selittää asian lyhyesti.
– Refers to a previously known person (he/she), without saying their role.

Finnish does not require a separate subject pronoun when a noun subject is present. You simply choose whether you want to use a noun (opettaja) or a pronoun (hän), depending on what you want to emphasize (role vs. person already mentioned).

Does opettaja always mean teacher in a school sense, or can it refer to other kinds of instructors?

Opettaja primarily means teacher, but it is fairly broad:

Schoolteacher (primary, secondary)
University teacher/lecturer (though lehtori, professori etc. also exist)
Someone who teaches a course or subject (e.g., a language teacher)

For more specific types, Finnish also has:

kouluttaja – trainer (often in workplaces, courses)
ohjaaja – instructor, director, coach (depending on context)
valmentaja – coach (sports, etc.)

In this isolated sentence, opettaja is best understood simply as the teacher, in the normal educational sense.