Can anyone explain me how to shorten the code I use?

[Mark Wieder]

William-

Here's my take on it:

local tErrMsg, x, tData, tErrs
local tFld, tBtn
put empty into tErrs
repeat with x=1 to 25
  put "Niveau van leerling" && x && "mag niet leeg zijn!" into tErrMsg
  put "naam" & x into tFld
  put "naam" & x & "nivo" into tBtn
  put field tFld && the selection of button tBtn into tData
  if field tFld is empty then
    if the selectedText of button tBtn is "..." then
      put empty into tData
    else
      --answer error tErrMsg
      put tErrMsg & cr after tErrs
    end if
  else -- field x is not empty
    if the selectedText of button tBtn = "..." then
      --answer error tErrMsg
      put tErrMsg & cr after tErrs
    end if
  end if
end repeat

...but I'm not sure exactly what you want the end result to be. Notice
that what you wrote could result in 25 error dialogs appearing on the
screen, one after another, which could be very annoying. I substituted
the accumulation of errors into a variable which could be checked
after the run:

if tErrs is not empty then
  answer error tErrs
end if

-- 
-Mark Wieder
 mwieder at ahsoftware.net