If I said that in three days' time it would be a Thursday, I am sure that most of you would have no difficulty telling me that today was a Monday. Try this one then. Yesterday was two days before Monday. What day is it today? Yes, you're right again. It's Sunday. Do you get the idea?

Now let's tackle a similar question from The National Mathematics Contest (1991) Paper:

Three days ago, yesterday was the day before Sunday. What day will it be tomorrow?
PLEASE HELP

The grass is not green; the grass is green.
Geometry is interesting; geometry is not interesting.
This rose is not white; this rose is white.
Two points determine a line; two points do not determine a line.
5+4≠90; 5+4=90.
All pigs are fat; all pigs are not fat.
Milk does not taste good; milk tastes good.
A line has no length; a line has length.
My dog has fleas; my dog doesn't have fleas.
A line does not have a midpoint; a line does have a midpoint.

To negate a statement, you make it mean the opposite thing.