How to say how careless you are to forget such an important thing! in Japanese
1)
そんなそんな(sonna) (adj-pn) such/like that/that sort of大切大切(taisetsu) (adj-na,n) important/valuable/worthy of careなな(na) (prt) connects adj to noun/indicates emotion or emphasis/command/prohibitionことこと(koto) (prt) particle indicating a command/particle indicating mild enthusiasm/particle indicating a gentle interrogative/particle used to soften a judgment or conclusionをを(wo) (prt) indicates direct object of action/indicates subject of causative expression/indicates an area traversed/indicates time over which action takes place/indicates point of departure or separation of action/indicates object of desire, like, hate,忘れno dictionary result, likely a conjigated verbno dictionary result, likely a conjigated verbるとno dictionary result, likely a conjigated verbno dictionary result, likely a conjigated verbはは(ha) (int) indicating a subject/yes/indeed/well/ha!/what?/huh?/sigh、、(、) Japanese comma君君(kun) (pn) you/buddy/pal/monarch/ruler/sovereign/masterはは(ha) (int) indicating a subject/yes/indeed/well/ha!/what?/huh?/sighずいぶんずいぶん(zuibun) (adv,adv-to) very/extremely/surprisingly/considerably/contemptible/reprehensible不注意不注意(fuchuui) (adj-na,n) carelessness/inattention/thoughtlessnessだだ(da) (aux) be/is/indicates past or completed action/indicates light imperativeねね(ne) (prt) indicates emphasis, agreement, request for confirmation, etc./is it so/hey/come on。。(。) Japanese period "."
