Why Can`t we say ' I did something' instead of saying 'I was doing something'?
'I did something' is a sentence in the simple past tense. This tense is used when the action is mentioned simply, without anything being said about its completeness or incompleteness of the action.
'I was doing something' is a sentence in the past continuous tense. This tense shows an action as incomplete or still going on in the past.