Find the sum and difference of successor and predecessor of $999$.

Step $1$; Find the sum and difference of successor and predecessor of $999$.

Step $2$; Find the successor of $999$

The successor of a number is $1$ more than the given number.

$\Rightarrow 999+1=1000$

Step $3$; Find the successor of $999$

The predecessor of a number is $1$ less than the given number.

$\Rightarrow 999-1=998$.

Step $4$; Find the difference of $999$ and $998$

Difference $=999-998=1$

Hence, the sum and difference of successor and predecessor is $1$