sydän is normally in the accusative case (which appears as either the nominative or genitive case depending on the context), but is inflected in the partitive case to express an ongoing or incomplete action or if used in the negative.
jonkun stands for "someone", which is inflected in the genitive case. If the person is expressed by using a personal pronoun, sydän receives a possessive suffix, and the pronoun can then be dropped if it is a first-person or second-person pronoun; see Appendix:Finnish possessive suffixes § Verb phrases for more:
