1.有多种说法.比较委婉的说法是,be afraid to do sth.例句：没受过训练的人因为怕出错而什么都不敢做.Those without training may be afraid to do anything for fear they would(will) do something wrong
根据不同语境,还有：
2.他不敢做这件事,真是个胆小鬼!He dare not do it!What a coward !
3.我要做失败者不敢做的事.I will do the work that a failure will not do .