Well doubt means 怀疑 in Chinese, so the first sentence is 我怀疑它不能起作用/工作，the second one is 我怀疑这不是个好主意。

我不确定这行部行

他们是直接翻译英语，我们要有中国味。 就如xiaolingzi说的，口语在这个情况不用怀疑。 第一句，我不知道它行不行。 第二句，我不知道它是不是好主意。

so the first answer was wrong. YY

Actually the expression is very similar with that in English - normally you can do direct translation. The two sentences in you question can be translated to: - 我怀疑它能工作/起作用 - 我怀疑它是个好主意