Once versus Used to

[Van Khanh]

What is the difference in meaning between these two sentences:

a. He was once the President of the USA.
b. He used to be the President of the USA.
In the two above sentences, if I use "the US" instead of "the USA", is it right?

Thanks
K

[Pamela]

Hi Van
Once means here at some time in the past.
He was once the President of the USA.
I think it's correct.
He used to be the President of the USA.(means that he was a President earlier, in the past)
I think they both are fine and similar in meaning.