what is the proper form for 싣다 in this conjugation?
싣다 turns into 실어요 but is it the proper form for 싣는다 as infinite form?

Question

Answer

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

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