If the action was ongoing or in progress in the past, the imperfecto (imperfect) form is used. In 1b and 2b, it is clear that the specific action is complete. 1b conveys that you attended a specific occurrence of a class (which one is implied from the context of the conversation, ...
First, let's look at an example of a u stem-changer. We'll use tener (or 'to have') to show it all. Tener's stem changes to tuv-. All conjugated, it looks like this: yo tuve nosotros tuvimos tú tuviste vosotros tuvisteis él/ella/usted tuvo ellos/ellas/ustedes tuvieron Other...