Past simple for completed actions in the past

The past simple tense is used for actions which are completed at a definite point in time in the past. This point can be implied or expressed with a time expression or a clause:

I bought a motorbike last week.
She was born in 1990.
Columbus discovered America. (i.e. in 1492)
How did it happen? (i.e. when it happened)
When I got home, I went to bed straight away.

Examples of time expressions that refer to a definite time in the past:

a year/two months/a few weeks/three days/etc. ago
at 5 o'clock
yesterday
earlier today/this week/this month
last week/month/year