Lokasuoja vaurioitui, mutta heijastin näkyy vielä.

Because they are the subjects of the sentence, and Finnish subjects are often in the nominative (the basic form).

• lokasuoja vaurioitui = the mudguard/fender was damaged / got damaged
• heijastin näkyy vielä = the reflector is still visible

Both verbs here are intransitive in this sentence, so there is no direct object forcing a different case.

Vaurioitui is the past tense of vaurioitua, which means to become damaged, to get damaged, or to be damaged.

So:

• vaurioitua = to get damaged
• vaurioitui = got damaged / was damaged

The ending -i marks the past tense, and the verb changes shape as part of normal Finnish conjugation. This is not something you can always translate piece by piece in a simple way, so it is best to learn:

• vaurioitua → vaurioitui

as a normal verb form pair.

No. It is not the Finnish passive.

It is a 3rd person singular past tense form of vaurioitua. The subject is still clearly present:

• Lokasuoja vaurioitui = The mudguard got damaged

A real Finnish passive would look different.

This verb is useful because it expresses that something underwent damage, without saying who caused it.

Because the sentence describes two different time frames:

• Lokasuoja vaurioitui = the damage happened earlier
• heijastin näkyy vielä = now, the reflector is still visible

So the sentence means something like:

• The mudguard was damaged, but the reflector is still visible.

This mixed tense is very natural when one event happened in the past, but its result is relevant in the present.

It is related, but not the same.

• nähdä = to see
  
  • this focuses on the person doing the seeing
• näkyä = to be visible / to be seen
  
  • this focuses on the thing that can be seen

So:

• Näen heijastimen = I see the reflector
• Heijastin näkyy = The reflector is visible / can be seen

In your sentence, näkyy means is visible or can still be seen.

Here vielä means still.

So:

• heijastin näkyy vielä = the reflector is still visible

Depending on context, vielä can also mean yet, more, or even, but in this sentence still is the best translation.

Because mutta means but, and in Finnish it normally introduces a new clause, so a comma is used before it.

• Lokasuoja vaurioitui, mutta heijastin näkyy vielä.

This is very similar to standard written English:

• The mudguard was damaged, but the reflector is still visible.

Yes, it is one compound word, which is very common in Finnish.

It is made from:

• loka = mud
• suoja = protection / shield

So lokasuoja literally means something like mud-protector, which corresponds to mudguard or fender.

Finnish uses compound nouns much more freely than English, so learners often need to get used to reading long words as smaller parts.

Yes, somewhat. Finnish word order is more flexible than English, although the neutral order here is very natural.

Neutral version:

• Lokasuoja vaurioitui, mutta heijastin näkyy vielä.

You could also say:

• Lokasuoja vaurioitui, mutta vielä heijastin näkyy.

But that sounds more marked or stylistically unusual in many contexts.

In everyday Finnish, the original order is the most natural because:

• subject first
• verb second
• vielä placed near the verb phrase

Because Finnish has no articles like a/an/the.

So:

• lokasuoja can mean a mudguard, the mudguard, or just mudguard, depending on context
• heijastin can mean a reflector or the reflector

The context tells you which one is meant. In this sentence, English naturally uses the.

Both are possible.

• was damaged sounds a bit more neutral
• got damaged often sounds a bit more natural in everyday English

Because vaurioitua focuses on the thing ending up damaged, got damaged is often a very good match.

So:

• Lokasuoja vaurioitui
  = The mudguard got damaged
  = The mudguard was damaged

Both are acceptable, depending on style.

Heijastin is in the nominative, because it is the subject of näkyy.

The verb näkyä does not take heijastin as an object here. The structure is:

• heijastin = subject
• näkyy = is visible

So Finnish is not saying someone sees the reflector. It is saying the reflector is visible.

That is why the noun stays in its basic form.

Then the second verb would also be in the past:

• Lokasuoja vaurioitui, mutta heijastin näkyi vielä.

That would mean something like:

• The mudguard was damaged, but the reflector was still visible.

So the difference is:

• näkyy vielä = is still visible now
• näkyi vielä = was still visible then

This is a useful contrast to notice.