"which shall go lost from him": to exclude (an object worth) less than a perutah, (whose loss he is not conscious of.) R. Yehudah says: "and you find it": to exclude (an object worth) less than a perutah.
"You shall not be able to ignore": This renders "ignoring" transgression of a negative commandment.
(Devarim 22:4) "You shall not see the ass of your brother or his ox fallen on the way and ignore them; lift up shall you lift up with him." "You shall not see the ass of your brother": This is a negative commandment. And elsewhere it is written (Shemoth 23:5) "If you see, etc.": That is a positive commandment.