doubtful
1.doubtful有时作“使人怀疑的”解，有时作“怀疑的”解，如Here is a doubtful point of grammar和I am doubtful of the construction of this sentence注意第二句里doubtful后面接of。
2.doubtful除了在否定句和疑问句里以外，后面通常接whether而不接that，如通常说It is not doubtful that he will come soon和Can it be doubtful that he will come soon? 但不说It is doubtful that he will come soon。最好把that改作whether。
3.It is doubtful whether he will come soon含有“他大约不会不久就来”的意思，It is doubtful whether he will not come soon含有“他大约不久就来”的意思。
4.doubtful（怀疑的）后面可以不用介词而直接以what，where，how等开首的短语或从句。例如：
- I am doubtful what to do with it.
- I am doubtful where he lives.
- I am doubtful how to proceed.
