自以为是: be opinionated; be self-righteous; believe sth. to be correct opinionately; bumptious; claim infallibility presumptuously; consider oneself always in the right; have a firm belief in one's
在实验中切不可粗心大意: do be careful with the experiment you're doing
火车切不可越过危险信号: a train must not pass a signal that is at danger
切不可作任何损人利己的事: never do anything to your profit at another's expense