靴下

How it works

No particle appears here — in casual speech 靴下(くつした) (socks) sits bare before the verb, with the object marker dropped. The verb 履き忘れる(はきわすれる) (forget to wear) is a compound (wear + forget), and it takes the casual past ending to become 履き忘れた(はきわすれた) (forgot to wear), so it reports something that already happened.