There is no rule when to employ which one particular, especially in situation of um. You must only learn which verb needs a person of these three phrases. Some verbs and phrases use um and there is no reason why: помощь, по́мощь, поддержка, помощник, подспо́рье, засту́пничество Auch wenn die Beispiele https://lassen98642.mysticwiki.com/804723/a_secret_weapon_for_für