I have always been a little confused as to when to use double negatives when writing (I am unsure if this is what they are termed in English, it is the term used in Mathematics).
E.g. when to use:
"I am not staying!"
Thus using two negatives -
...as opposed to:
"I am going!"
Here using a single positive -
Both sentences begin with
"I am..." which infers that the person is going to do something, but then this can either be followed by two negatives, so to speak;
"...not staying" which cancel each other out and mean
going, or it can simply be followed by the word
In Mathematics a double negative is equal to a positive, and this also seems to be the case in the English language (and perhaps others). However when to use either one seems up for debate.
There are plenty of other examples, e.g.
"I am not unregistered." or
"I am registered."