Opaste näyttää retkeilyreitin ja korkeuseron.

Opaste means a sign, guide sign, or wayfinding sign—something that gives information or directions.

It is similar to kyltti, but not always exactly the same:

• opaste = an informational or directional sign
• kyltti = a sign/placard more generally

So in this sentence, opaste suggests a sign that informs the reader about the hiking route and elevation difference.

Here näyttää means shows or displays.

So the structure is:

• Opaste = the sign
• näyttää = shows
• retkeilyreitin ja korkeuseron = the hiking route and the elevation difference

A useful thing to know is that näyttää has more than one common meaning in Finnish. It can also mean to look/seem, depending on the structure:

• Se näyttää hyvältä. = It looks good.
• Opaste näyttää reitin. = The sign shows the route.

In your sentence, it is clearly the shows/displays meaning.

Because they are both objects in the sentence, and in this kind of affirmative sentence Finnish often uses the total object form in the singular, which usually looks the same as the genitive singular.

So:

• reitti → reitin
• korkeusero → korkeuseron

This is very common in Finnish. English learners often first learn -n as a genitive ending, but in sentences like this it is functioning as the form used for a complete object.

The idea here is that the sign shows:

• the whole hiking route
• the whole elevation difference

not just some unspecified part of them.

Those would be partitive forms:

• retkeilyreittiä
• korkeuseroa

The partitive is often used when something is:

• incomplete
• ongoing in an indefinite way
• only part of something
• unspecified in quantity

But here the sentence presents the route and the elevation difference as complete, definite pieces of information shown by the sign. That is why the sentence uses the total object forms:

• retkeilyreitin
• korkeuseron

Compare the general contrast:

• Opaste näyttää reitin. = The sign shows the route.
• Opaste näyttää reittiä. = The sign is showing a route / some route / route information in a more partial or less natural sense here.

So in this sentence, the -n forms are the natural choice.

It is a compound word plus an ending:

• retkeily = hiking / trekking / outdoor excursion activity
• reitti = route
• retkeilyreitti = hiking route / hiking trail
• retkeilyreitin = the object form used here

Finnish uses compound words very freely, so long words like this are completely normal.

You can think of it as:

retkeily + reitti + -n

It comes from:

• korkeus = height, elevation
• ero = difference

Together:

• korkeusero = difference in elevation / elevation difference

And in the sentence:

• korkeuseron = object form of korkeusero

This is another very typical Finnish compound noun.

Finnish does not have articles like a/an and the.

So a noun like opaste can mean:

• a sign
• the sign

and the exact meaning depends on context.

The same applies to:

• retkeilyreitin = a/the hiking route in context
• korkeuseron = an/the elevation difference in context

Finnish usually leaves definiteness to be understood from the situation rather than marking it with a separate word.

The basic word order is:

Subject + Verb + Object

So:

• Opaste = subject
• näyttää = verb
• retkeilyreitin ja korkeuseron = object phrase

This is a very normal Finnish sentence pattern and is fairly close to English:

• The sign shows the hiking route and the elevation difference.

Finnish word order can be moved around for emphasis, but this version is neutral and natural.

Because they are two coordinated objects joined by ja (and), and both belong to the same grammatical role in the sentence.

So:

• retkeilyreitin
• ja
• korkeuseron

Both are things that the sign shows, so both appear in the same object form.

This is very common in Finnish:

• Näen talon ja auton. = I see the house and the car.
• Opaste näyttää retkeilyreitin ja korkeuseron.

Not exactly.

• reitti = route, itinerary, path in a broad sense
• polku = path, trail, footpath

A retkeilyreitti is a hiking route/trail, often understood as a marked route for hiking.
A polku is more the physical path itself.

So retkeilyreitti is the better word here if the sign is giving route information.

A rough pronunciation guide is:

RET-kei-ly-rei-tin

A few useful points:

• Stress is always on the first syllable in Finnish.
• ei is a diphthong, somewhat like the vowel sound in English day, but shorter and cleaner.
• Each written sound is pronounced fairly consistently.

If you break it up:

• ret
• kei
• ly
• rei
• tin

Finnish pronunciation is usually much more regular than English spelling.

Not in this sentence.

Although näyttää can mean looks/seems, that meaning usually appears in a different structure, often with an adverbial or complement:

• Opaste näyttää uudelta. = The sign looks new.
• Se näyttää hyvältä. = It looks good.

In your sentence, näyttää has a direct object:

• retkeilyreitin ja korkeuseron

That makes it clearly mean shows/displays, not looks.

The dictionary forms are:

• opaste → opaste
• näyttää → näyttää
• retkeilyreitin → retkeilyreitti
• ja → ja
• korkeuseron → korkeusero

So if you look them up, you would search for:

• opaste
• näyttää
• retkeilyreitti
• ja
• korkeusero