Sometimes there are Latin verbs that in English are "to be ____". Some of these are taceo, -ere "to be quiet", gratulor, gratulari "to be thankful", gaudeo, -ere "to be glad", irascor, -sci "to be angry". Obviously, you can just use the imperative of the verb in these cases.
T. is quite correct about the adjectives, though volitive subjunctive is more common with esse than the imperative.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae