be on firm ground
be sure of one´s facts; be secure in one´s position, esp in a discussion 对事实确信无疑; 立场坚定(尤指在讨论中). a firm `hand strong discipline or control 严明的纪律; 牢固控制
That boy needs a firm hand to help him grow up. 那个男孩需要严加管教才能有所长进. have, etc a firm/tight hold on sth => hold2.