Just to really complicate matters (stay with us!) there is more than one past tense! We start by taking a look at the so-called perfect tense.
When to use it: For describing what has happened, what someone has done or what happenend in the past.
How to form it: You need to remember that there are two parts to a perfect tense verb - the auxiliary verb and the past participle.
The auxiliary verb shows who or what did the action and the past participle shows what happened.