In the medieval hymn, Dies Irae, there is a stanza:
Rex tremendæ majestatis,
Qui salvandos salvas gratis,
Salva me, fons pietatis.
Which I guess is intended to be understood as:
King of awesome majesty,
Who freely saves those who are to be saved,
Save me, fountain of justice.
However, I don't understand how salveo is getting conjugated here. First of all, the imperative of salveo is salve, not salva, I thought. Secondly, the present indicative of salveo I thought was salvet ('he saves'), not salvas. Neither salvas nor salva even appear in standard conjugations of salveo.
What is the explanation for these forms?