Learn Hardcore Finnish: Luen uuden sanan ääneen, jotta korva tottuu ääniin ja suu harjoittelee. - I read the new word aloud so that the ear gets used to the sounds and the mouth practices.

luen is the 1st person singular form of lukea (to read), so it already means I read.
Adding minä is possible (Minä luen uuden sanan…) but it gives extra emphasis, like I read the new word… (not someone else).

So Luen uuden sanan… is the normal, neutral way to say I read a new word….

What tense and meaning does luen have? Could it also mean “I am going to read”?

luen is the present tense of lukea:

It can mean I read (habitually):
Luen uuden sanan ääneen = I read a new word aloud (whenever I encounter one).
It can also refer to an action happening now:
I am reading a new word aloud.
In some contexts it can imply a near future:
I’ll read a new word aloud (soon).

Finnish doesn’t have a separate continuous tense, so luen covers I read and I am reading.

What case is uuden sanan, and why isn’t it just uusi sana?

uuden sanan is in the genitive singular, and together it functions as the object of luen:

uusi sana = a new word (basic form, nominative)
uuden sanan = the new word as a total object (genitive)

In Finnish, the object of a verb often appears in:

Genitive (like uuden sanan) when the action is seen as complete / whole:
- Luen uuden sanan. = I read the whole word (once, completely).
Partitive (like uutta sanaa) when the action is ongoing, incomplete, or repeated in a “partial” sense:
- Luen uutta sanaa. = I am (in the process of) reading a new word / working on it.

Also note agreement:

uuden (genitive of uusi)
sanan (genitive of sana)

The adjective must match the noun in case, number, and (here) in singular.

What exactly does ääneen mean, and why does it appear with lukea?

ääneen is the illative singular form of ääni (sound, voice). Literally, it means “into (a) sound/voice”, but in this expression it’s idiomatic:

lukea ääneen = to read aloud, to read out loud

So Luen uuden sanan ääneen is best translated as:

I read the new word aloud.

You can think of it as a set phrase: lukea ääneen ≈ read out loud.

Why are there both ääneen and ääniin in the same sentence? What’s the difference?

They look similar but are different in number and in how they’re used:

ääneen = illative singular of ääni
- comes from the expression lukea ääneen (to read aloud).
ääniin = illative plural of ääni
- used with the verb tottua (to get used to), which takes its complement in the illative.

So:

Luen uuden sanan ääneen = I read the new word aloud.
jotta korva tottuu ääniin = so that the ear gets used to the sounds.

In English you need the preposition to; in Finnish the illative ending -iin on ääni does that job.

What does jotta mean, and how is it different from että?

jotta is a conjunction that introduces a purpose/result clause:

jotta ≈ so that, in order that

In the sentence:

Luen uuden sanan ääneen, jotta korva tottuu ääniin ja suu harjoittelee.
= I read a new word aloud so that the ear gets used to the sounds and the mouth gets practice.

että is more neutral and usually introduces a statement or reported speech:

Tiedän, että opit nopeasti. = I know that you learn quickly.

You can’t simply replace jotta with että here, because että doesn’t normally express purpose.
jotta connects your action (I read) with the goal/result (the ear gets used, the mouth practices).

Shouldn’t korva and suu have a possessive, like korvani (my ear) and suuni (my mouth)?

In Finnish, body parts are often used without possessive endings when it’s clear from context whose they are.

korva tottuu = the ear gets used (to…)
suu harjoittelee = the mouth practices

Because I am the one reading, it’s naturally my ear and my mouth. Adding possessives:

jotta korvani tottuu ääniin ja suuni harjoittelee

is grammatically correct but sounds more formal or emphasized than necessary in everyday speech. The simple korva / suu is the most natural here.

Why is it korva tottuu ääniin and not something like korva tottuu ääneen?

Two things are going on: the verb’s government and number.

Verb pattern:
tottua (to get used to) takes its complement in the illative case:
- tottua + illative
- tottua ääniin = to get used to the sounds
Plural meaning:
ääniin is illative plural of ääni:
- ääneen = into a sound (singular)
- ääniin = into sounds (plural)

Here we’re talking about getting used to various sounds of the language, so the plural ääniin (to the sounds) is used.

What form is tottuu, and how does it relate to tottua?

tottua is the basic (dictionary) form = to get used to, to become accustomed.
tottuu is 3rd person singular present tense:
- hän tottuu = he/she gets used (to…)
- korva tottuu = the ear gets used (to…)

So korva tottuu ääniin literally = the ear gets used to the sounds.

You can think of the pattern:

tottua johonkin = to get used to something
where johonkin is in the illative (here: ääniin).

Why is there no object after harjoittelee? What is the mouth “practising”?

harjoitella can be used:

transitively (with an object):
- harjoittelen sanoja = I’m practising words.
- suu harjoittelee ääniä = the mouth practises sounds.
or more intransitively, meaning something like to practise / to get practice in general.

In … ja suu harjoittelee, the idea is:

the mouth gets practice
the mouth is practising (speaking these sounds)

The object (sounds, speaking) is understood from context (we are reading sounds/words aloud) and can be left out naturally in Finnish.

Why is it korva tottuu ääniin ja suu harjoittelee, not korva tottuu ja suu harjoittelee ääniin?

Because ääniin belongs specifically with tottuu:

tottua ääniin = to get used to the sounds
harjoitella ääniä would need partitive plural (ääniä) to mean practise sounds.

If you said:

…korva tottuu ja suu harjoittelee ääniin

it would sound ungrammatical or at least very strange, because:

harjoitella ääniin is not a normal combination (wrong case for the object).
The case -iin signals direction (into sounds), not a direct object of practising.

So:

korva tottuu ääniin (ear gets used to the sounds)
suu harjoittelee (mouth practises / gets practice).

Could this also be Luen uutta sanaa ääneen…? What would change?

Yes, you could say:

Luen uutta sanaa ääneen…

but the meaning shifts slightly:

Luen uuden sanan ääneen…
= I read the new word aloud (one whole word, as a complete act).
Luen uutta sanaa ääneen…
= I am in the process of reading a new word aloud / working on it (focus on the ongoing activity, not on completing the entire word).

In practice, for a short word, uuden sanan is more natural here, since you clearly complete the action of reading the word.

Is korva tottuu ääniin ja suu harjoittelee a general statement, or about this specific action?

The present tense here can express a general, habitual result:

Luen uuden sanan ääneen, jotta korva tottuu ääniin ja suu harjoittelee.
= Whenever I encounter a new word, I read it aloud so that in general my ear gets used to the sounds and my mouth gets practice.

So it describes a habit/strategy: you regularly do this (read aloud) with the purpose of training your ear and mouth over time, not just in this one specific instance.