这句话用英语该怎么说啊？

应该是用将来时态的。
I am afraid that due to the poor knowledge I have of some professional English vocabularies, I might not be able to achieve a good score in the up-coming examination.

worry 是忧愁，是真正的情感忧愁。而此处，说话人的忧愁害怕不过是客套，是为了说明一个自我评价而表达的态度。不是情绪上的忧愁。

I fear I for some professional English vocabulary does not know, cause I can't very well in the examination to obtain good results.

I am afraid that due to the lack of understanding for English vocabulary, I am unable to acheive good results for the coming examinations.

I'm afraid i can not perform very well in the exam due to my lack of professional English vocabulary.

第一个翻译的很好,语法正确.不过把my shortcoming on改成the lack of感觉会更好.别人的功劳,你该取他的翻译.