Learn Finnish: Kahvissa on sokeria. - There is sugar in the coffee.

The basic word is sokeri, which ends in a single short vowel (i). Words ending in a single vowel usually take the -a or -ä partitive ending. We choose -a instead of -ä because of vowel harmony: the word sokeri contains a back vowel (o), so the ending must also use the back vowel a.

Why is sokeria in the partitive case here?

Sugar is an uncountable substance (a mass noun). In Finnish, when you say "there is" some amount of a substance, that word goes into the partitive case. This shows you are talking about an unspecified amount of it (some sugar), rather than all the sugar in the world.

Why does the sentence start with Kahvissa instead of a word for "there"?

Finnish does not have a direct translation for the English filler word "there" in "there is" sentences. Instead, to say "there is sugar in the coffee", Finnish starts with the location (Kahvissa), followed by the verb (on), and ends with the thing that is there (sokeria).