Basically if you are talking about something that you could OR could not do, then you use to words.
ie. I can reply to your question. OR
I can not reply to your question.
Because you can leave out the second word 'not' then you would use two words.
If you are talking about something that you could not do no matter how hard you try, then you would use one word.
ie. I cannot ever know what everyone is thinking.
I 'can' know what everyone is thinking would be a false sentence. There for dropping the word 'not' makes it irrelevant.
**OK told you it was dificult to put into writing. I'm going to give you a link to a website that better explains it**
Attention: NOTHING on this site may be reproduced in any fashion whatsoever without explicit consent (in writing) of the owner of said material, unless otherwise stated on the page where the content originated. Search engines are free to index and cache our content. Users who post their account names or personal information in their questions have no expectation of privacy beyond that point for anything they disclose. Questions are otherwise considered anonymous to the general public.