"I have seen him swim."
번역:나는 그가 수영하는 것을 본 적이 있습니다.
"I have seen him swim" is a sentence meaning in past tense and only describing that you have seen him swimming. The word "to" does not fit in here because it is not talking about going to someone or something. A sentence like "Did you go to him" or "I went to him" would be correct.