iubeo

Iubeo, iubere, iussi, iussum, second conjugation. The b of the present stem vanishes into a double s in the perfect and supine, which is why iussit looks unrelated to iubet until you know to expect it.

The construction is the thing that separates readers who have been taught from readers who have not. Iubeo takes an accusative and infinitive: iubeo te ire, "I order you to go". Impero, by contrast, takes a dative of the person and ut with the subjunctive. If the person ordered is not named, Latin turns the infinitive passive: pontem fieri iussit, "he ordered a bridge to be made". The neuter participle iussum is an order, iussa in the plural, and the ablative absolute iussu Caesaris means "on Caesar's orders". Grammarians' jussive, the commanding subjunctive, is named straight off it.