If I remember correctly,
B2Remembering something, but with a degree of uncertainty.
To the best of my recollection,
B2Remembering something as accurately as possible.
Back then,
B2Referring to a specific time in the past.
At that time,
B2Referring to a specific moment in the past.
Previously,
B2Before a particular time or event.
Earlier,
B2Before the current time.
In the past,
B2Referring to a period of time before the present.
Once upon a time,
B2Beginning a story about the past, often informal.
Years ago,
B2Referring to a time many years in the past.
Some time ago,
B2Referring to a period of time in the past, less specific than 'years ago'.
A while back,
B2Referring to a period of time in the past, informal.
During that period,
B2Referring to a specific period of time in the past.
Throughout my childhood,
B2Referring to the entirety of someone's childhood.
In my youth,
B2Referring to someone's younger years.
When I was younger,
B2Referring to a time in the past when someone was younger.
Prior to that,
B2Before a particular event or time.
Up until then,
B2Until a particular point in the past.
Beforehand,
B2Before a particular event or action.
At one point,
B2At a specific time in the past.
It happened that...
B2Describing an event that occurred.
That time when...
B2Referring to a specific past event.
From my experience,
B2Based on past events.
Looking back on it,
C1Reflecting on a past event.
In retrospect,
C1Considering something after it has happened.