for certain and uncertainly we can use the following expressions

neutral:
i'm sure..
l'm quiet sure..
l've no doubt about...
informal:
yes
sure
absolutely for certain
formal:
it's quite certain..
that is my conviction

FOR UNCERTAINTY we can use the following expressions
neutral:
l'm not sure.
l'm not really sure
l can't decide..
informal:
l can't say for certain
l couldn't say
formal:
there is surely some doubt....