Kuulin oudon äänen.

Kuulla is the infinitive, meaning to hear.
Kuulin is a finite verb form, meaning I heard.

It breaks down like this:

• kuul- = verb stem
• -i- = past tense marker
• -n = I / first person singular

So:

• kuulla = to hear
• kuulin = I heard

Finnish usually does not need the pronoun minä (I) here, because the ending -n already shows who did the action.

Because Finnish often leaves out subject pronouns when the verb ending already makes the subject clear.

In kuulin, the ending -n already tells you the subject is I. So:

• (Minä) kuulin oudon äänen.

Both are possible, but usually minä is omitted unless you want emphasis or contrast.

For example:

• Minä kuulin oudon äänen, mutta sinä et kuullut.
  I heard a strange sound, but you didn’t.

Because the adjective must match the noun in case and number.

The basic form is:

• outo = strange, odd

But in this sentence the noun äänen is in the object form used here, so the adjective must also change:

• outo ääni = a strange sound
• oudon äänen = a strange sound (as the complete object of the verb)

So oudon is the form of outo that agrees with äänen.

This is a very important Finnish pattern:
adjectives agree with the nouns they describe.

The basic form is:

• ääni = sound

In the sentence, the noun is the object of the verb kuulin. Finnish changes object nouns depending on how the action is viewed.

Here, äänen is the total object form in the singular. It suggests the sound is being treated as a whole event or whole object: I heard a strange sound.

So:

• ääni = sound
• äänen = sound (as a total object in this sentence)

This is very common in Finnish object marking.

This is a classic learner question.

Formally, äänen looks exactly like the genitive singular form. But in this sentence, its job is that of the object.

In traditional grammar, people often say that singular total objects like this are in the accusative, even though for most nouns the form is identical to the genitive.

So you can think of it like this:

• form: looks like genitive singular
• function: object of the verb

For a beginner, the most useful idea is:

• äänen is the normal singular total object form here

That is more practical than worrying too much about the label.

Because kuulin oudon äänen presents the hearing as a complete event: you heard the sound.

Finnish often uses the total object when the action is seen as complete, bounded, or affecting the whole object.

With verbs of perception like kuulla (to hear), the total object is common when you mean you heard the sound as a whole event:

• Kuulin oudon äänen. = I heard a strange sound.

This contrasts with the partitive in some situations.

Outoa ääntä is the partitive form. It gives a different nuance.

Compare:

• Kuulin oudon äänen.
  I heard a strange sound.
  → a whole sound/event; a specific sound was heard

• Kuulin outoa ääntä.
  I was hearing strange sound / I heard some strange-sounding noise / I heard something strange-sounding
  → more indefinite, partial, ongoing, or less bounded

The partitive often appears when the object is:

• indefinite or unbounded
• only partly affected
• ongoing or incomplete in some sense

So the sentence you were given, Kuulin oudon äänen, is the more natural choice if you mean one distinct strange sound.

The adjective outo has the stem oudo- in many inflected forms.

So:

• basic form: outo
• stem used for inflection: oudo-
• plus -n gives oudon

This is not something you can always predict perfectly from English, so it is worth learning the stem together with the basic form.

You will see the same pattern in other forms too:

• outo
• oudon
• outoa
• outona

So oudon is not random; it comes from the inflectional stem oudo-.

Similarly, ääni uses a different stem when it is inflected.

• basic form: ääni
• stem: ääne-
• plus -n gives äänen

So:

• ääni = sound
• äänen = sound in this inflected form

Many Finnish nouns ending in -i behave like this, though not all in exactly the same way. It is often best to learn both the dictionary form and a few common stem forms.

This is the most neutral word order:

• Kuulin oudon äänen.

Finnish word order is more flexible than English, but changing it usually changes the emphasis or information structure.

For example:

• Kuulin oudon äänen.
  Neutral: I heard a strange sound.

• Oudon äänen kuulin.
  More marked; emphasizes the strange sound

• Äänen kuulin, en nähnyt mitään.
  Emphasizes the sound

So the given order is the normal, straightforward choice.

A rough pronunciation guide:

• Kuulin ≈ KOO-lin
• oudon ≈ OW-don
  with ou as a diphthong, somewhat like the vowel glide in go
  • oo
• äänen ≈ AEE-nen
  where ä is like the vowel in British cat for many speakers, but longer here

A few useful points:

• uu in kuulin is a long vowel
• ää in äänen is also a long vowel
• Finnish stress is normally on the first syllable:
  • KUUlin
  • OUdon
  • ÄÄnen

Long vowels matter in Finnish, so it is good to notice the difference between u and uu, ä and ää.

Yes, depending on context.

Finnish has no articles like a and the, so oudon äänen can sometimes be translated either way:

• I heard a strange sound
• I heard the strange sound

If there is no earlier context, English usually prefers a strange sound.
If the sound has already been mentioned, the strange sound may be the better translation.

So Finnish leaves that distinction to context rather than articles.