The following verbs have irregular forms of the imperative.
Irregular verbs
avoir = to have
être = to be
savoir = to know
vouloir = to want *
tu / you
aie (Have!)
sois (Be!)
sache (Know!)
veuille (May you xxx!!)
nous / we
ayons (Let's have!)
soyons (Let's be!)
sachons (Let's know!)
veuillons (Let's ...)
vous / you
ayez (Have!)
soyez (Be!)
sachez (Know!)
veuillez (May you xxx!)
These verbs form the imperative with the subjonctif.
In other Roman languages like Spanish and Italian this is actually the standard form of the (negative) imperative. This is logical, as the the subjonctif does express a kind of demand. However, grammar is not always logical and therefore, in French we only have the exceptional case of the subjonctif for imperative.
* The verb vouloir is a special case, which we will discuss more in detail in the following page.