The French past imperative is very rare, because its usage is restricted to a single situation: it gives a command for something that must be done before a certain time.

   Aie écrit ce rapport demain.
   Have this report written by tomorrow.

If you obey the instructions in the above example, when tomorrow comes the report will already be written, so the writing of it will be in the past, ergo, the past imperative.

If you use the regular imperative, Écris ce rapport demain, the report won't yet be written when tomorrow rolls around: in accordance with the command, you will be writing it tomorrow. On the other hand, the careful use of a preposition can make all the difference - you could just say Écris ce rapport avant demain and avoid the past imperative altogether - probably another reason that it is so rare.

   Soyez partis à midi.
   Leave / Be gone by noon.

   Ayons fini les devoirs à 7h00.
   Let's have our homework done by 7:00.

The past imperative is kind of similar in nuance to the past infinitive, except that it indicates a command rather than a statement of fact.

Because the past imperative is so rare, there's really no need to learn how to use it, but you should be able to recognize it. Go on to page 2 for past imperative conjugations.