Correction in english

 I have to use "so that" in an essay can you help me : is that correct?

"Vincent hadn’t dared to ask me the question so that he will never know why I had done that…"

The way you have your sentence is more of a double negative.

You could use something like "Vincent needs to ask me the question so that he will know why I had done that", but it sounds a little bit awkward.  

You could also use "Vincent needs to ask me the question so that he will know why I did it."

Ben

My Blog

Haha if an English man says it's awkward. Oh boy, this is awkward!

Them English people can speak English way better than the Americans. 

I know I'm not a native speaker, but even to me it sounds odd.

I'd say – Vincent didn't dare to ask me the question, therefore, he will never know why I've done that.

I was just wondering if the "double negative" restriction also applies to sentences where the second negative is placed in another clause.  

http://www.yourdictionary.com/library/reference/word-definitions/define-double-negative.html

Yea, but I also appreciate that there is more to English than grammar rules. "Sounding natural" and style are also important. So, sentences that appear to be grammatically correct can often be improved upon. Even if you are confident of your grammar, getting a second opinion helps.  

http://www.grammar-quizzes.com/8-9.html

http://www.accu-assist.com/grammar-tips-archive/GrammarTip_double-negatives.htm