50 do you think that I should enter for the competition 为什么不可以用whether

这里主句是疑问句。而宾语从句是陈述句。所以用that 而不用whether.要分清是对主句还是从句的疑问。

Do you think...后的宾语从句须为陈述句.如：Do you think he can finish it?Do you think he
shouldn't do it like that?Do you think 的意思本身就是“你是否认为...”,而whether (还有if)在宾语
从句中的意思就是“是否”,两者连用,意义重复.顺便一提：whether引导的宾语从句可以看作是
由一般疑问句或选择疑问句转变而来.e.g.
Does he like tea?I don't know whether/if he likes tea.
Does he like tea or coffee?I don't know whether he likes tea or coffee.