PS C:\> $a = "x is $($x++)"

PS C:\> $a

x is

PS C:\> $a = "x is $x++"

PS C:\> $a

x is 2++

PS C:\>

Your second question is not that complicated:

In the first example it does not count up because foreach of the three "rounds" you are just printing the value of $a which is "x is1".

In the second example in each of the three rounds you have the expression x++ in the "foreach" and thus x gets incremented by 1 each round.

Hope this helps

Daniel