THE PROGRESSIVE FORMS OF A VERB INDICATE THAT SOMETHING IS HAPPENING or was happening or will be happening. When used with the past, the progressive form shows the limited duration of an event: "While I was doing my homework, my brother came into my room." The past progressive also suggests that an action in the past was not entirely finished. (Compare "I did my homework." to "I was doing my homework.") This is even more evident in the passive progressive construction: "He was being strangled in the alley" suggests an action that was not finished, perhaps because the act was interrupted by a good citizen, whereas the simple past "He was strangled in the alley" suggests an action that was finished, unfortunately.
A neat categorization of the uses of the progressive can be found on the page describing the "To Be" Verb.
The progressive forms occur only with dynamic verbs, that is, with verbs that show qualities capable of change as opposed to stative verbs, which show qualities not capable of change.* For instance, we do not say, "He is being tall" or "He is resembling his mother" or "I am wanting spaghetti for dinner" or "It is belonging to me." (We would say, instead: "He is tall," "He resembles his mother," "I want spaghetti," and "It belongs to me.") The best way to understand the difference between stative and dynamic verbs is to look at a table that lists them and breaks them into categories and then to build some sentences with them, trying out the progressive forms to see if they work or not.
These categories and lists are derived from Randolph Quirk and Sidney Greenbaum's A University Grammar of English (used with the publisher's permission).
Activity Verbs I am begging you. I was learning French. They will be playing upstairs..Virtually identical in meaning to simple tense forms:I beg you. I learned French. They will play upstairs. abandonaskbegcalldrink eathelplearnlistenlook at playrainreadsayslice throwwhisperworkwrite
Process VerbsThe corn is growing rapidly. Traffic is slowing down.Virtually identical in meaning to simple present tense forms:The corn grows rapidly. Traffic slows down. changedeteriorate growmature slow down widen
Verbs of Bodily Sensation "I feel bad" and "I am feeling bad" are virtually identical in meaning. ache feel hurt itch
Transitional Events Verbs Progressive forms indicate the beginning of an event,as opposed to the simple present tense."She was falling out of bed [when I caught her]" as opposed to "She falls out of bed every night.