대통령에

대통령에

"To the president."

"대통령에" here indicates the recipient of an action, specifying that the action of sending a gift is directed towards "대통령" or "the President."

대통령에게 선물을 보냈습니다.

I sent a gift to the President.

"대통령에 대해" uses "에 대해" to express about regarding the "대통령" or "the President."

우리는 대통령에 대해 논의하고 있습니다.

We are discussing about the President.

"대통령에 의해" uses "에 의해" to indicate that the law was enacted through the action of the "대통령" or "the President."

대통령에 의해 법이 공표되었습니다.

The law was announced by the President.