When addressing someone in the second person informal, the imperative is formed with the verb stem, without the -st du ending. For most verbs, it is optional to add an -e as well. However, for verbs that require an -e when conjugating them with du (arbeiten-arbeitest, antworten-antwortest), the -e is mandatory.
Sei still! Be quiet!
Tue* das nicht! Don’t do that! *here, the -e is optional.
Beantworte* die Frage! Answer the question! *here, the -e is required.
With the exception of e-ie and e-i stem-changing verbs (sehen, helfen, etc.), the vowel change is not present in the imperative form: