Have to use % to "go back to the start"

UsingForSyntax.ps1

$String = 'SQL Server'
for ($i = 0; $i -lt $String.Length; $i++ ) {
     [PSCustomObject]@{
         'This Char' = $string[$i]
         'Prev Char' = $string[$i -1]
         'Next Char' = $string[($i +1) % $String.Length]
     }
 }