In the past we also have a progressive form and a simple one and the rules are similar. Again, when the action was only temporary the verb takes the past progressive. Consider, “When I came into class the teacher was telling a joke”. The teacher’s telling of the joke was only temporary. Note as well that the teacher was in the act of telling the joke or continuing the action. In other words, the past progressive, as with the present progressive, does not express a finished action.
We use the simple past to speak about an action that was completed. By examining again the above example it is becomes obvious that “to come” is in the simple past because it expresses an action that was finished.
In the following example it can be seen that the simple past can interrupt the action that was in progress. Take, “A taxi hit Mary while she was crossing the street”. As a result of being hit by the taxi, Mary never finished crossing the street.
Now for some of the exceptions: With some verbs there is no indication that there is any action at all. These are, therefore, non-progressive. Examples are: know, see, like, etc. Check you textbook for a more exhaustive list. In “Bradley knows the answer,” just because he knows it does not imply that he is doing anything at all.
Some verbs can have a progressive meaning and another one that is non-progressive. Consider, “Mr. Cash has a dictionary,” and “Miss Universe is having dinner now”. In the former sentence “have” means possess. This does not imply that Mr. Cash is doing any action what-so-ever. However, when have means to eat, it clear that there is an action intended.
Sometimes an action can occur repeatedly or habitually but can take the progressive tense. In such instances, the speaker is expressing strong emotion as in, “Your’re always interupting me”. In such cases something may have been done that was not intended to arouse the anger expressed or the situation was unplanned. “Mrs. Potter always meets her daughter up after school and takes her home,” is an example of a planned meeting. However, “Mrs. Potter is always meeting her daughter’s teacher at the supermarket,” is the case of an unplanned meeting.
REFERENCE: How English Works; A Grammar Practice Book; by Michael Swan & Catherine Walter; Oxford University Press; 1997.