Reittimerkintä on punainen.

Because Finnish writes compound nouns as a single word.

Reittimerkintä is made of:

• reitti = route, trail
• merkintä = marking, mark, sign

So reittimerkintä is a compound noun, literally something like route-marking or trail marker.

This is very common in Finnish. Where English often uses two words, Finnish often uses one compound word.

It is in the nominative singular, which is the basic dictionary form for a noun.

In this sentence, reittimerkintä is the subject:

• Reittimerkintä = the route marker / trail marker
• on = is
• punainen = red

So grammatically, it is the thing being described.

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

So reittimerkintä can mean:

• a route marker
• the route marker

Which one is meant depends on context.

This is one of the biggest differences from English. Finnish usually leaves definiteness unstated unless the context makes it clear.

On is the 3rd person singular present tense of the verb olla, which means to be.

So:

• minä olen = I am
• sinä olet = you are
• hän on = he/she is
• reittimerkintä on = the route marker is

In this sentence, on works just like English is.

Punainen is the basic dictionary form of the adjective red.

Here it is a predicate adjective, meaning it comes after on and describes the subject:

• Reittimerkintä on punainen.
• The route marker is red.

With a singular nominative subject like reittimerkintä, the adjective is also in the nominative singular, so punainen is exactly the form you would expect here.

Yes, but this is a slightly different situation from an adjective directly in front of a noun.

Compare:

• punainen reittimerkintä = a red route marker
  
  • here the adjective is directly attached to the noun
• Reittimerkintä on punainen = the route marker is red
  
  • here the adjective is a predicate after on

In both cases, with a singular nominative noun, the adjective appears in nominative singular.

So in this sentence, punainen matches reittimerkintä naturally.

No. Finnish has no grammatical gender.

That means adjectives do not change for masculine, feminine, or neuter, because Finnish does not have that kind of system.

So punainen is just punainen, regardless of whether you are talking about:

• a person
• an object
• a place

Any changes in the adjective come from things like case and number, not gender.

Yes, Finnish word order is somewhat flexible, but Reittimerkintä on punainen is the most neutral and natural order.

This order is:

• subject
  • verb
    • predicate adjective

If you change the order, the meaning may stay similar, but the emphasis changes. For example, putting punainen first would sound marked or contrastive, like emphasizing that it is specifically red.

So for a basic statement, Reittimerkintä on punainen is the standard form.

A rough pronunciation guide for an English speaker would be:

• reittimerkintä ≈ RAYT-ti-MER-kin-tah
• on ≈ on with a short o
• punainen ≈ PU-nai-nen

A few useful points:

• Finnish stress usually falls on the first syllable.
• Double letters matter:
  
  • tt in reitti- is pronounced longer than a single t
• ä is not the same as English a. It is a front vowel, somewhat like the vowel in British cat, but cleaner and shorter.
• Finnish pronunciation is usually very regular: words are pronounced quite close to the way they are written.

Many Finnish adjectives and nouns end in -nen. It is a very common ending.

In the dictionary form, red is punainen. When the word is declined, the stem changes:

• punainen = red
• punaisen = of the red / red's
• punaista = red (partitive)

So -nen words are an important pattern to learn early. In this sentence, though, you are just seeing the basic form: punainen.