thần bí huyền bí khó hiểu 
mysterious like: khó hiểu → →
zwielichtig ☊ 
mờ ám, khó hiểu ☊ 
可疑的 ☊ kěyí de zwielichtig ☊ mờ ám, khó hiểu ☊ ☊ That name sounds awfully familiar. ☊ awfully ☊
to clear up a mystery/difficulty/misunderstanding ☊ clear sth up ☊
The instructions on the box are very confusing. ☊ confusing ☊
a very confusing experience ☊ confusing ☊
The report was fairly incomprehensible. ☊ fairly ☊
It was difficult to understand the local dialect. ☊ local ☊
He has a complete mental block (= difficulty in understanding or remembering) when it comes to physics. ☊ mental ☊