that은 해석상 의미가 큰 비중을 차지하지 않기 때문에 생략이 가능합니다.
that이외의 접속사는 생략 되었을 때 의미의 손실을 가져오기 때문에 생략이 불가능합니다.
I didn't know that he was there.
Do you know when he was there.
The important thing is where he went there.
I know how he went there.
I know why he went there.
I know which people went there.
I know who went there.
I know with whom you go there.
I know what you say.
I wonder if you went there.
I wonder whether you went there.
참고 : 문장과 문장을 연결하기 위해 종속접속사를 사용하면 부사절과 명사절이 만들어집니다.