Orberg in LLPSI Cap XIV has Syra telling Marcus: "Noli tristis esse quod hic intus cubas: immo laetare te non inclusum esse in cubiculo ut fratrem tuum!
Earlier in LLPSI we had (somwhere I'm sure) One character saying to another: "Laeta es!"
Orberg explains laetare as : the imperative of laetari.
Now Prior and Wholberg's 501 Latin Verbs he doesn't mention this... But I remember that (usually? / always?) the present infinitive of a verb is also the passive infinitive of that verb.
Do I have that right?