Commentaire sur Le Lévitique 4:27

ואם נפש אחת תחטא, And if anyone of the common people sin through error, etc. The letter ו at the beginning of the word ואם connects this paragraph to the rules established in the previous paragraphs concerning the details of the procedures.

ואם נפש אחת תחטא בשגגה מעם הארץ, if an ordinary citizen had committed an inadvertent sin, a likely event; here the Torah describes the sin as חטא, whereas when speaking of the King it describes it as אשם, in order to warn him at the same time to confess and do teshuvah before offering his sin offering. In both instances, as well as in all sacrifices described as asham, part of the sacrificial meat is eaten by the priests. The eating of those parts of the sacrifice by the priests is an essential ingredient in the atonement process for the guilty parties.

ואשם, “and he has become guilty;” he found out himself that he had committed an inadvertent sin.