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.

be keen about, be keen on, be known as, be known for, be known to, be likely to, be located in, be lost on, be loyal to, be made of, be made up of, be made up of..., be married to sb., be native to, be no match for, be noted as, be noted for, be occupied in, be off one's head, be on a diet, be on hands and knees, be on one's mind, be on the advance, be on the drink, be opposed to, be out for, be out of employment, be out of one's mind, be out of step, be out of style, be pleased with, be popular with, be prejudiced against..., be qualified to, be related to, be relevant to, be reluctant to, be remarkable for, be representative of, be responsible for,