Lokasuoja irtosi eilen matkalla.

Because Finnish makes compound nouns very freely.

lokasuoja = loka + suoja

• loka = mud, slush
• suoja = protection, guard

So the word literally means something like mud-protector. In natural English, that becomes mudguard or fender, depending on context and region.

Writing it as one word is completely normal in Finnish.

The basic verb is irrota.

Here, irtosi is:

• past tense
• 3rd person singular

So it means came off, came loose, or became detached.

A useful comparison:

• irtoaa = comes off / becomes detached
• irtosi = came off / became detached

So in this sentence, lokasuoja irtosi means the mudguard came off.

Yes, usually.

irrota is typically intransitive here: something comes loose or comes off.

So Lokasuoja irtosi suggests:

• the mudguard became detached
• it wasn’t necessarily presented as a deliberate action by someone

If you wanted to say that someone removed it, Finnish would more naturally use a transitive verb such as irrottaa.

So this sentence focuses on what happened to the mudguard, not on who did it.

It is in the nominative singular.

That is the basic dictionary form, and here it works as the subject of the sentence:

• lokasuoja = the thing that came off
• irtosi = came off

So structurally, this is a very straightforward subject + verb combination.

Because Finnish has no articles.

There is no direct equivalent of English a or the. Finnish usually leaves that idea to be understood from context.

So lokasuoja can mean:

• a mudguard
• the mudguard

depending on the situation.

Also, Finnish often leaves out possessive ideas like my or our if the context makes them obvious. In a sentence like this, people often understand that it means something like the mudguard on the bike/car/etc. we’re talking about.

Because matkalla is the adessive form of matka.

• matka = trip, journey
• matkalla = on the trip / on the way / while traveling

This is a very common Finnish expression. Literally, the -lla ending often has meanings like on, at, or in the course of something.

So matkalla is best understood here as:

• on the way
• during the journey
• while traveling

It is idiomatic Finnish, not something you should translate too literally word for word.

Because matkalla is the normal expression for being on the way or traveling.

For example:

• Olen matkalla kotiin. = I’m on my way home.

The form matkassa usually means something different, depending on context, such as along with or included with something. So in this sentence, matkalla is the natural choice.

Because Finnish does not need to say it if it is not important.

matkalla by itself can simply mean:

• on the way
• during the trip

If the destination mattered, Finnish could add it:

• matkalla kotiin = on the way home
• matkalla töihin = on the way to work

But in Lokasuoja irtosi eilen matkalla, the point is just that it happened during the journey, not where the journey was headed.

eilen is a time adverb meaning yesterday.

It tells you when the event happened.

So the sentence has three main pieces:

• Lokasuoja = subject
• irtosi = verb
• eilen matkalla = time/circumstance information

Unlike many Finnish words, eilen does not take a special case ending here. It is just the normal adverb form.

Yes. Finnish word order is more flexible than English word order.

The neutral order here is perfectly natural:

• Lokasuoja irtosi eilen matkalla.

But other orders are also possible, depending on emphasis:

• Eilen matkalla lokasuoja irtosi.
  Emphasizes when/under what circumstances
• Matkalla lokasuoja irtosi eilen.
  Possible, though less neutral
• Irtosi lokasuoja eilen matkalla.
  More marked, often stylistic or context-dependent

Finnish often moves words around to highlight what is new, important, or contrastive information. So word order is not random, but it is more flexible than in English.

A simple way to see it is:

• subject: Lokasuoja
• verb: irtosi
• time adverb: eilen
• circumstantial expression: matkalla

So the sentence is basically:

[Something] + [happened] + [when] + [in what situation]

This is a very common Finnish sentence type. Once you recognize the subject and the verb, the rest often consists of adverbs or case forms giving extra information such as time, place, or circumstance.