Learn Finnish: Pienessä kulhossa on sokeria. - There is sugar in the small bowl.

This is an existential sentence, which introduces what exists in a certain place (translated as "There is..."). In these sentences, uncountable mass nouns like sugar must be in the partitive case (sokeria) because we are talking about an undefined amount of it.

Why does the sentence start with the location Pienessä kulhossa?

Finnish doesn't have a direct equivalent to the English dummy subject "There" (as in "There is"). Instead, to say "there is something somewhere", Finnish sentences typically start with the location (Pienessä kulhossa), followed by the verb (on), and end with the subject (sokeria).

Both the adjective and the noun have the "in" ending, but why are the endings slightly different (-ssä vs -ssa)?

Adjectives must match the case of the noun they modify, so both take the inessive ("in") ending. However, because of Finnish vowel harmony, the front-vowel word pieni takes the front-vowel ending -ssä (pienessä), while kulho contains back vowels (u, o) and therefore takes -ssa (kulhossa).

If the subject were plural instead of the mass noun sokeria, would the verb on change to the plural ovat?