The Genitive Case

In English we can indicate possession in two ways - using the word "of" or putting an apostrophe-s on the end of the possessor. So for example we say, "the Gulf of Finland" or "Peter's book". Deciding which to use is easy for a native English speaker. There are even situation when either works - you can say "the roof of the house" or "the house's roof".

So in English we can modify a noun to indicate possession, which means it is a case. In fact, it's called the Genitive Case.

In Finnish the genitive case is made by adding an -n to the end of the noun:

talo - house
talon - of the house, for example talon katto - the house's roof, or the roof of the house

Leena - popular Finnish woman's name
Leenan - Leena's, for example Leenan auto - Leena's car.

suomi kieli - the Finnish language
suomen kielen - of the Finnish language, for example suomen kielen fonetiikka - the phonetics of the Finnish language

Note that in the last example, the i at the end of each word turned into an e. This is just something that happens to words ending in i that are put in the genitive, and has to be learnt.