Commands (Formal, USTED), regular 1

How to form usted commands — Take the yo present indicative, drop the -o, and add the "opposite vowel" ending: -ar verbs → -e; -er/-ir verbs → -a. So hablo → hable, como → coma, vivo → viva. Any irregularity in the yo form carries through: tengo → tenga, pongo → ponga.

Yo-form irregulars affect usted commands — Because usted commands derive from the yo present tense, any verb irregular in the yo form will be irregular in the command: hacer → hago → haga, decir → digo → diga, traer → traigo → traiga, salir → salgo → salga. There are no surprises if you start from the yo form.

Usted vs ustedes commands — The usted command ends in -e/-a (singular); the ustedes command adds -n: hable → hablen, coma → coman, viva → vivan. In Latin America, ustedes commands replace both formal and informal plural commands (there is no vosotros form in everyday Latin American speech).