Imperative Mood

For -ER verbs:

The imperative conjugations for nous and vous are the same as the present indicative, minus the final s for the tu form.

Verbs which are conjugated like -ER verbs (the indicative tu ends in -es), such as ouvrir and souffrir, follow the same rules as -ER verbs and drop the final s for tu. 

The imperative conjugations for all regular and most irregular -IR and -RE verbs are the same as the present indicative conjugations (meaning keep the s for tu).

Four irregular imperative verbs:
avoir
   (tu) aie
   (nous) ayons
   (vous) ayez

être
   (tu) sois
   (nous) soyons
   (vous) soyez

savoir
   (tu) sache
   (nous) sachons
   (vous) sachez

vouloir
   (tu) veuille
   (nous) n/a
   (vous) veuillez

http://french.about.com/od/grammar/a/imperative_2.htm