How do you say “be!” in Italian?

Sii come l’acqua — “be like water.” That sii is the tu imperative of essere, and it’s irregular enough to stop learners cold: not sei, not essi, but sii, with two i’s. Its neighbours are just as odd: abbi (avere), sappi (sapere), vogli (volere). They’re the handful of commands that don’t come from the present tense at all.

These four are frozen old subjunctives pressed into service as commands, which is why the plurals go back to normal: siate, abbiate, sappiate, vogliate. The negative breaks differently again — “don’t be” is non essere, the bare infinitive, never non sii. The whole clip is one long chain of imperatives: impara a cantare senza stancarti mai… mormora, insinuati, balza o zampilla… sii l’acqua che dorme, che corre, che gioca.