싣다 is ‘ㄷ’ 불규칙 ; ‘ㄷ’ Irregular verb When a verb stem ending in ‘ㄷ’ is followed by an ending beginning with a vowel, ‘ㄷ’ changes to ‘ㄹ’. Verb(ㄷ ending) + vowel -> ㄷ changes to ㄹ 싣(다) + 어 -> 실어 싣(다) + 으 -> 실으 싣는다 is 해라체 indicative non-past form 실어요 is 해요체 indicative non-past form You can see full conjugation table at https://en.wiktionary.org/wiki/%EC%8B%A3%EB%8B%A4