Learn Polish: Jemu zależy na członkostwie w partii. - He cares about membership in the party.

Jemu is the long, emphasized form of the dative pronoun for on (he). You must always use the long form when a pronoun is the very first word in a sentence. Here, it also adds a slight contrastive emphasis, implying that he is the one who cares about it, perhaps as opposed to someone else.

Why do we use the dative Jemu zależy instead of a nominative structure like On zależy?

When the verb zależeć means 'to care about' or 'to matter', it requires an impersonal construction. The person who feels the emotion is the indirect object in the dative case (here, jemu), and the thing they care about is introduced by na plus the locative case. You can think of it as literally translating to 'It matters to him.'

How is the abstract noun członkostwo built from the root noun członek (member)?

First, the mobile e drops out of członek to give us the stem członk-. Then we attach the abstract noun suffix -stwo. Because the stem ends in a hard consonant cluster, the linking vowel -o- is inserted to make pronunciation easier, resulting in the collective/abstract noun członkostwo (membership).

What happens to the -stwo suffix when it declines in this sentence?