There are only two imperative forms: present and past. You can forget past because it is very rare. Personally, I have nearly never heard it. Just to know, it is build using auxiliary être and avoir. (Aie rangé tes affaires quand j'arrive).
Main imperative characterics is to exist only to persons tu, nous and vous. It can express an order, an advice or a suggestion.
As a general rule, it is identical to present indicative except there is no subject. The only exception is to remove the finale s from the second person to all verbs finishing in e (verbs in -er included aller and a few third group verbs such as cueillir). Exception in front of en and y where the s is keeping just for sound (see grammar note for this rule).
- Cueille des fleurs! - Va te coucher! - Allons-y! - Mangez des pommes!