会计师的无条件满意的保证使得“顾客”相信由会计师操刀的纳税申报表一定是精确的.你翻译的很好.但最好把“clients”译成“客户”或“委托人”
An accountant's unconditional guarantee of satisfaction 作主语
lead作谓语
believe that the tax returns prepared by the accountant is certain to be accurate作补语
that the tax returns prepared by the accountant is certain to be accurate宾语从句,作 believe 的宾语
the tax returns prepared by the accountant 做从句的主语,其中prepared by the accountant 作后置定语修饰t he tax returns