The verbs être, avoir and savoir are irregular in Impératif:
être | avoir | savoir | |
tu | sois | aie | sache |
nous | soyons | ayons | sachons |
vous | soyez | ayez | sachez |
On their own, they're not terribly useful, but they are essential to express some commands.
See also these other lessons on Impératif :
Conjugate regular verbs in L'Impératif (imperative)
Conjugate être, avoir, savoir in L'Impératif (imperative)
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