En halua hukata avainta matkalla kotiin.

Because Finnish often leaves out subject pronouns when they are obvious from the verb form.

Here, en already tells you the subject is I:

• en = I do not
• et = you do not
• ei = he/she/it does not, and so on

So En halua... naturally means I do not want... without needing minä.

You could say Minä en halua hukata avainta matkalla kotiin, but it sounds more emphatic, like I don’t want to lose the key.

In Finnish, negation uses a special negative verb, and that verb changes according to the person.

The forms are:

• en = I do not
• et = you do not
• ei = he/she/it does not
• emme = we do not
• ette = you all do not
• eivät = they do not

So because the subject is I, the correct form is en.

After the negative verb, the main verb appears in a special form called the connegative.

Compare:

• Haluan = I want
• En halua = I do not want

So in negative sentences, Finnish does not use the normal personal ending on the main verb. The person is already shown by the negative verb en.

That is why:

• haluan becomes halua
• not en haluan

Because haluta means to want, and when one verb follows another in this kind of structure, the second verb is usually in the first infinitive.

So:

• haluan hukata = I want to lose
• en halua hukata = I do not want to lose

Here:

• halua = want
• hukata = to lose

This is the normal pattern for want to do something in Finnish.

Hukata usually means to lose, especially in the sense of misplacing something or no longer knowing where it is.

It is commonly used for physical objects such as:

• keys
• phone
• wallet

So hukata avain means to lose a key or to misplace a key.

A related verb is menettää, which also means to lose, but it is broader and can be used for things like:

• losing a job
• losing a game
• losing hope

In this sentence, hukata is a very natural choice for a key.

Because the object of a negative sentence is usually in the partitive case.

So:

• avain = key
• avainta = key (partitive singular)

Since the sentence is negative:

• En halua hukata avainta = I do not want to lose the key / a key

This is a very important Finnish rule:

• in many affirmative sentences, the object may be in the genitive or nominative
• in negative sentences, the object is typically in the partitive

So the negation is the main reason for avainta.

Because avain belongs to a noun type whose stem changes when cases are added.

Its main forms are:

• avain = key
• avaimen = of the key / the key (genitive)
• avainta = key (partitive)
• avaimessa = in the key
• avaimet = keys

So this word does not just add endings to avain unchanged. The stem alternates, and you simply have to learn its pattern.

This type is fairly common in Finnish, so it is worth getting used to these stem changes.

Matkalla means on the way, during the journey, or while traveling, depending on context.

It comes from:

• matka = trip, journey, way
• matkalla = on a trip / on the way

In this sentence, matkalla kotiin means on the way home.

Grammatically, matkalla is in the adessive case, which often has meanings like on, at, or during. But in this expression, it is best learned as the set phrase:

• matkalla kotiin = on the way home

Because kotiin is the illative form of koti, and the illative often expresses movement into or to somewhere.

So:

• koti = home
• kotona = at home
• kotiin = to home / homeward

Finnish uses a direction case here, because the idea is movement:

• matkalla kotiin = on the way home

This is very natural Finnish. English uses home without a special ending, but Finnish marks the direction clearly with -iin here.

Yes, very much so.

Matkalla kotiin is a common Finnish phrase meaning:

• on the way home

Similar expressions include:

• matkalla töihin = on the way to work
• matkalla kouluun = on the way to school
• matkalla kauppaan = on the way to the store

So it is useful to learn matkalla + destination in a directional case as a common pattern.

Yes. Finnish word order is more flexible than English word order, because case endings show grammatical relationships.

The neutral order here is:

• En halua hukata avainta matkalla kotiin.

But other orders are possible for emphasis, for example:

• Matkalla kotiin en halua hukata avainta.
• Avainta en halua hukata matkalla kotiin.

These all mean roughly the same thing, but the emphasis changes.

The original sentence sounds neutral and natural:

• first the negated verb phrase
• then the object
• then the adverbial phrase matkalla kotiin

It can mean either, depending on context.

Finnish does not have articles like a and the, so avainta by itself does not tell you whether the meaning is:

• a key
• the key
• sometimes even my key or the key in question, if the context makes that clear

So you understand it from the situation, not from an article.

In many real contexts, this sentence would probably be understood as I don’t want to lose my/the key on the way home, but grammatically Finnish does not state that directly here.