More to get a feeling of the severity of this error, how bad does it sound when someone makes the mistake of not separating a separable verb?
"Ich bereite mich auf eine Prüfung vor."
"Ich vorbereite mich auf eine Prüfung."
Now, 1) is correct (I hope!), and 2) is wrong.
But... how wrong? Would a native speaker understand it? Is it irritating to the ears of a native speaker? To what degree?
Greatly appreciated would be an equivalently messed-up and incorrect variant of the English equivalent of 1), which is:
1') I prepare myself for an exam.
How would 2') sound like in (broken) English? To get a feeling of the severity of the mistake.
While this is not maybe truly a question regarding correct German, it is though a question about (broken) German. And the fact is that many beginners struggle with separable verbs (forget to separate, misplace the prefix, or forget that they still have a prefix to place).
Other examples of broken German/broken English equivalents would be greatly appreciated.