Jarrujen ja vaihteiden pitää toimia, ja heijastimien pitää näkyä.

In this sentence, pitää is being used in a different common meaning: must / have to.

A very common Finnish pattern is:

X:n pitää tehdä jotain = X must do something

So here:

• jarrujen ja vaihteiden pitää toimia = the brakes and gears must work
• heijastimien pitää näkyä = the reflectors must be visible

So yes, pitää can mean hold/keep, but in this structure it means must / have to.

Because in this necessity construction, pitää normally stays in the 3rd person singular form.

So Finnish says:

• minun pitää mennä = I must go
• lasten pitää odottaa = the children must wait
• jarrujen pitää toimia = the brakes must work

Even if the logical subject is plural, the verb is still pitää, not pitävät.

This is one of the first things English speakers notice, because English would normally make the verb agree with the subject, but Finnish does not do that here.

They are in the genitive plural.

In the pitää + infinitive structure, the thing that has to do something is often put in the genitive:

• jarrujen = of the brakes
• vaihteiden = of the gears
• heijastimien = of the reflectors

A very literal way to think of it is:

• of the brakes and gears must function
• of the reflectors must be visible

That sounds unnatural in English, but it helps explain the Finnish grammar.

So this is not a possession meaning like the brakes’ in English. It is just the normal case used with this kind of must/have to construction.

Because after pitää, Finnish uses the first infinitive.

The pattern is:

pitää + infinitive

So:

• pitää toimia = must work
• pitää näkyä = must be visible

If you said toimivat or näkyvät, those would be finite verb forms like work / are visible, and they would not fit after pitää in this structure.

Compare:

• Jarrut toimivat. = The brakes work.
• Jarrujen pitää toimia. = The brakes must work.

Here toimia means to function, to operate, or simply to work.

So jarrujen ja vaihteiden pitää toimia means that the brakes and gears need to be in working order.

This is a very common Finnish verb for machines, systems, devices, and equipment:

• Puhelin toimii. = The phone works.
• Internet ei toimi. = The internet isn’t working.

Näkyä means to be visible or to be seen.

So:

heijastimien pitää näkyä means the reflectors must be visible.

This is important because näkyä is not an active verb like show. It describes something that can be seen.

So the idea is not that the reflectors are doing the showing; it is that people should be able to see them.

This is a very common question, because the words look similar.

• näkyä = be visible / be seen
• näyttää = show, or sometimes look / seem

Examples:

• Talo näkyy hyvin. = The house is clearly visible.
• Kartta näyttää tien. = The map shows the road.
• Hän näyttää väsyneeltä. = He/She looks tired.

So in your sentence, näkyä is correct because the reflectors need to be visible, not show something.

Because it is joining two different kinds of things in two different places.

First ja joins two nouns:

• jarrujen ja vaihteiden = the brakes and the gears

Second ja joins two whole clauses:

• jarrujen ja vaihteiden pitää toimia
• heijastimien pitää näkyä

So the sentence structure is:

[The brakes and gears must work], and [the reflectors must be visible].

Because these are really two separate clauses, and each one has its own logical subject:

1. jarrujen ja vaihteiden pitää toimia
2. heijastimien pitää näkyä

Repeating pitää makes the structure clear and natural.

English can also repeat must in the same way:

• The brakes and gears must work, and the reflectors must be visible.

So the Finnish sentence is built very neatly in two parallel parts.

Yes. A rough literal gloss is:

jarrujen = of the brakes
ja = and
vaihteiden = of the gears
pitää = must
toimia = function / work
ja = and
heijastimien = of the reflectors
pitää = must
näkyä = be visible

So the grammar is roughly:

[of the brakes and gears] must [function], and [of the reflectors] must [be visible].

That is not good English, but it shows how Finnish is putting the sentence together.

They are all plural genitives, but the exact endings are not identical because Finnish nouns belong to different word types.

In this sentence:

• jarru → jarrujen
• vaihde → vaihteiden
• heijastin → heijastimien

So they all mean of the ... / the ...’s in form, but here they are being used because of the pitää construction.

This is a good reminder that Finnish plural genitive formation has several patterns, so you usually need to learn the stem changes along with the noun.