Learn Hardcore Finnish: Huomaan virheen heti, kun luen viestin uudestaan. - I notice the mistake immediately when I read the message again.

In Finnish, a comma is usually placed before a subordinate clause. Here kun luen viestin uudestaan is a kun-clause (a subordinate time clause), so it gets separated by a comma:

Huomaan virheen heti, kun luen viestin uudestaan. = I notice the mistake as soon as I read the message again.

In everyday writing you may sometimes see the comma omitted, but the standard rule is to include it.

What exactly does heti kun mean—just when, or as soon as?

Heti = immediately / right away, and kun = when. Together, heti kun commonly means as soon as (i.e., immediately at the moment when something happens).

So the sentence has the “as soon as” meaning, not just a neutral “when”.

Why is virhe in the form virheen?

Virheen is the object of huomaan (I notice). The ending -n here marks a total object (often called genitive object in the singular), suggesting the mistake is noticed as a whole/complete item:

Huomaan virheen. = I notice the (specific) mistake.

If you used the partitive virhettä, it would suggest an incomplete/ongoing process or “some mistake” in a less bounded way, depending on context:

Huomaan virhettä… (less natural here; could imply “I’m noticing something wrong / some error” in an ongoing way)

Why is viesti in the form viestin in luen viestin?

For the same reason: viestin is the object of luen (I read), and in many contexts “read the message” is treated as a total/complete action → total object viestin.

If you said luen viestiä, that often implies:

you’re reading some of it, or
you’re in the middle of reading, or
it’s more ongoing/indefinite.

So luen viestin uudestaan strongly suggests “I read the message again (from start to finish / as a complete read-through).”

What verb forms are huomaan and luen?

Both are present tense, 1st person singular:

huomaan = I notice (dictionary form: huomata)
luen = I read (dictionary form: lukea)

Finnish present tense covers both “present” and sometimes “near-future” meanings depending on context.

Does this sentence describe a habit, a general truth, or something happening right now?

It can be either, depending on context, because Finnish present tense is flexible.

Common readings:

General/habitual: I notice the mistake immediately whenever I read the message again.
Specific/this situation (in the right context): I notice the mistake right away as soon as I read the message again.

If you wanted to emphasize a one-time completed event in the past, you’d typically use past tense:

Huomasin virheen heti, kun luin viestin uudestaan. = I noticed the mistake as soon as I read the message again.

Why is there no word for I (like minä)?

Finnish verb endings already show the person, so the subject pronoun is usually unnecessary:

Huomaan… already means I notice…

You might include minä for contrast or emphasis:

Minä huomaan virheen heti… = I (as opposed to someone else) notice the mistake immediately…

Can the word order be changed?

Yes, Finnish word order is fairly flexible, but changes can shift emphasis.

Neutral/common:

Huomaan virheen heti, kun luen viestin uudestaan.

Possible variations:

Huomaan heti virheen, kun luen viestin uudestaan. (slightly more emphasis on heti)
Heti kun luen viestin uudestaan, huomaan virheen. (fronts the time clause; common in writing)

The core grammar stays the same.

What does uudestaan mean, and how is it different from uudelleen or taas?

Uudestaan = again / anew, very common in speech and writing.

Related options:

uudelleen also means again, often feels a bit more formal/neutral.
taas often means again or back, but can also mean on the other hand / in turn, so it’s more context-dependent.

In this sentence, uudestaan is a natural choice: luen … uudestaan = I read … again (re-read).

Is kun the only option here? What about kunnes or jolloin?

Here kun is the normal word for “when/as soon as” in this structure.

kunnes means until and would change the meaning:
- Huomaan virheen vasta, kunnes… doesn’t fit the intended “as soon as” idea.
jolloin is closer to at which point / when, but often feels more formal and a bit different in timing nuance:
- Huomaan virheen heti, jolloin luen… is not natural for this meaning.

So for “as soon as,” heti kun is the standard pattern.

How do you pronounce the key parts, and where is the stress?

Finnish stress is almost always on the first syllable of a word:

HUO-maan
VIR-heen
HE-ti
LU-en
VIE-stin
UU-des-taan

Also note Finnish has long vs short sounds (length matters), though nothing in this sentence has an especially tricky “double vowel/double consonant” spelling beyond normal clear vowels.