ago-since

a / an

ago / since / for


1	ago는 과거에 발생한 것에 대하여 말하고자 year, month, day, time 같은 단어 다음에 사용되며 ...전에 라는 의미로 쓰인다.
1	A long time ago, this area was a forest.
2	That happened years ago.
3	I moved here eight years ago.
4	A long time ago, dinosaurs lived in this area.

2	since는 특정 시점이나 사건과 함께 ...이후 (지금까지), ...로부터 (지금까지) 라는 의미로 사용된다.
1	I've been waiting since three o'clock.
2	I've been waiting since 2:30.
3	She has been happier since she moved here.
4	Since leaving college, he's lived with his parents.

I've been waiting from 2:30. (x)
I've been waiting since 2:30. (o)


3	for는 과거나 미래에서 기간(amounts of time)을 나타내는 단어와 함께 ...동안 이라는 의미로 사용된다.
1	I have been waiting for three hours.
2	The building will last for years.

A long time ago, dinosaurs lived
in this area.

I've been waiting since twelve o'clock.

I have been waiting for three hours.