确信(què xìn) - to be certain, to be convinced in Mandarin

确信 (què xìn) is belief with the doubt taken out. It sits a step above 相信: you can 相信 a friend's excuse while privately wondering, but 确信 says you have no reservations left.

It normally takes a whole clause as its object rather than a single noun: 我确信他会来, 她确信自己没有做错. The negative is 不能确信 or 无法确信 more often than a flat 不确信.

The trap is that it looks like a stronger 相信 and gets used everywhere. It is more written and more emphatic than English "I'm sure", which in casual speech is usually 我觉得 or 肯定. Keep 确定 for settling a fact or a plan (时间确定了吗？), 肯定 for the emphatic adverb (他肯定知道), and 确信 for a conviction you would defend. Note too that as a verb of certainty it resists 很 - say 我很确信 and you will sound translated.