‘before’ and ‘ago’

Before means at some unknown time before now. It does not indicate when. We generally use the present perfect tense with before.

Example:

I have never seen him before.

Ago means at a certain time before now. Ago tells us how long before the present time something happened. It tells us when and gives us a time or a date. Because we are referring to a specific time in the past, the simple past is used.

Example:

a. She left about an hour ago.

b. I saw her for the first time at a conference five years ago.
NOTE:

Ago always counts back from the present time. Note that if we are counting back from a past time, ‘before’ or ‘earlier’ or ‘previously’ are used, not ‘ago’:

Example:

I met him on a flight to London in December 2010 when he told me that he had permanently moved to Tokyo four years before.
(= 6 years ago; counting back from a past time)