①I doubt whether I should take part in that activity.
②I'm unsure whether to take part in that activity.
③I can't decide whether to participate in that activity.
AI解析句①Whether后面要接言完整的句子，是因为要做doubt的宾语，所以要用完整的句子, 而不能用whether to do的结构.And如果这样的话，那③句decide后面是whether引导的宾语却用了whether to do的结构，怎么被解释呢？
②句为什么能用whether to do的结构呢？是因为做unsure的定语的原因所以可以用to do的结构？那就不能用whether+完整句子的结构了？
什么情况下必须要用whether to do or whether+完整句子的判断依据。请大神科普