Sometimes in VBScript I get errors if I don't use parentheses around function arguments and other times I get the error "can't use parentheses when calling a subroutine" when I *do* use them. What's the problem?

Part of what is going on is VBScript distinguishing between different kinds of procedures (Sub or Function). Whether there is a return value being part of that distinction. The upshot is this: You will need parentheses if you are calling a function that returns a value and you are using the return value, or if you are using the 'Call' keyword. For example:

' Parentheses can't be used here. Disregarded return value
' makes this a subroutine call.
crt.screen.WaitForString "string", 5

' Parentheses needed here. Return value makes this a
' function call.
Dim result
result = crt.screen.WaitForString("string", 5)

' Parentheses are always needed when using 'Call'
Call crt.screen.WaitForString("string")
Call crt.screen.WaitForString("string", 5)

' With one argument both of these forms are legal.
crt.screen.WaitForString "string"
crt.screen.WaitForString("string")