The imperative is really second person only; the other forms are supplied by the subjunctive.
So the third person guardi is the same as the present subjunctive. The second person ending comes from Latin, notwithstanding guardare is not of Latin origin.
In the imperative, amare has ama/amate (same as Latin), leggere has leggi/leggete (lege/legite), salire has sali/salite (same as Latin).
In the present indicative, the Latin amas has become ami; the desinence -i is common for the second person in all conjugations. The present subjunctive of amare is ami/ami/ami/amiamo/amiate/amino (Latin amem/ames/amet/amemus/ametis/ament).