Fix issue #1. \xmark behaves differently in math environment.

Reported by Georg Regitnig. Thanks!
Fix issue #3. "meisten" becomes "meistens".

Reported by Georg Regitnig. Thanks!
Fix issue #5. "SI-Präfixe" becomes "SI-Präfixen".

Reported by Georg Regitnig. Thanks!
Fix issue #4. Modal verb requires "werden".

Reported by Georg Regitnig. Thanks!