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