Questions & Answers about Seinässä on kosteutta.
Because -ssa / -ssä is the Finnish inessive case, which usually means in or inside something.
- seinä = wall
- seinässä = in the wall / inside the wall
The ending is -ssä rather than -ssa because Finnish vowel harmony requires front vowels here. Since seinä contains ä, the ending also uses ä.
So seinässä is not a separate word for in + wall; Finnish expresses that idea by adding a case ending to the noun.
On is the 3rd person singular form of the verb olla, which means to be.
- minä olen = I am
- sinä olet = you are
- hän on = he/she is
- se on = it is
In this sentence, on is used in an existential structure, something like there is / there exists in English.
So:
- Seinässä on kosteutta = There is moisture in the wall.
Even though English often uses there is, Finnish usually just uses on.