Hyräilin melodiaa koko kotimatkan.

Hyräilin is the 1st person singular past tense of hyräillä, which means to hum.

You can break it down like this:

• hyräillä = to hum
• hyräili- = past tense stem
• -n = I

So hyräilin means I hummed or I was humming.

Because Finnish usually does not need a subject pronoun when the verb ending already shows the person.

Here, the ending -n tells you the subject is I. So:

• hyräilin = I hummed / I was humming

You could add minä for emphasis:

• Minä hyräilin melodiaa koko kotimatkan.

But in a normal sentence, leaving minä out is more natural.

Finnish does not have a separate past progressive form like English was humming.

Instead, the ordinary past tense often covers both:

• I hummed
• I was humming

The exact English translation depends on context. In this sentence, the combination of:

• melodiaa (partitive object)
• koko kotimatkan (the whole trip home)

makes the action feel ongoing over a period of time, so I was humming is a very natural translation.

Melodiaa is the partitive singular of melodia.

Finnish often uses the partitive object when the action is seen as:

• ongoing
• uncompleted
• repeated
• not focused on a finished whole

So hyräilin melodiaa suggests something like:

• I was humming a melody
• I was humming some tune
• I kept humming the melody

If you said melodian instead, that would present the melody more as a complete whole, as if you hummed the entire melody through.

Also, the form itself is regular:

• melodia → melodiaa (partitive singular)

Kotimatka is a compound word:

• koti = home
• matka = trip, journey

So literally it is something like home-trip, but in natural English it means:

• trip home
• journey home
• way home

Finnish uses compounds very often, and they are written as one word.

Koko kotimatkan means for the whole trip home or all the way home.

This is a common Finnish time-duration pattern:

• koko päivän = all day / the whole day
• koko yön = all night / the whole night
• koko matkan = the whole trip / all the way

Here:

• koko = whole, entire
• kotimatkan = genitive singular of kotimatka

So the phrase marks the full duration of the action.

In expressions of duration with koko, Finnish often uses the noun in the genitive singular.

So:

• koko päivä is not correct in this sentence
• koko päivän = for the whole day
• koko kotimatkan = for the whole trip home

This is just a very common Finnish structure for saying that something lasted through an entire span of time.

Because koko and kaikki are not used in the same way.

• koko = the whole / entire of one thing
• kaikki = all in a plural or collective sense

Here there is one trip home, treated as a single complete unit, so:

• koko kotimatkan = the whole trip home

You would not use kaikki here.

Because kotimatkalla and koko kotimatkan mean slightly different things.

• kotimatkalla = on the way home, during the trip home
• koko kotimatkan = for the whole trip home, all the way home

So:

• Hyräilin kotimatkalla.
  = I was humming on the way home.

• Hyräilin melodiaa koko kotimatkan.
  = I was humming a melody the whole way home.

The second one emphasizes that the humming continued for the entire duration.

In this kind of expression, koko usually stays in its basic form, while the following noun takes the case ending.

So you get:

• koko päivän
• koko yön
• koko kotimatkan

That is normal Finnish usage.

Yes. Finnish word order is fairly flexible because case endings show what each word is doing.

For example, you could also say:

• Koko kotimatkan hyräilin melodiaa.

That version puts more emphasis on the whole trip home.

The original:

• Hyräilin melodiaa koko kotimatkan.

is a natural, neutral way to say it.

Because Finnish has no articles.

So melodiaa could mean:

• a melody
• the melody
• just melody/tune

The exact meaning depends on context, not on a separate word like a or the.