I was told that I can use Past Continuous with the expressions measuring time.
I was waiting at the station for 3 hours.
What is the exact difference between this sentence and the following?
I waited at the station for 3 hours.
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Something to keep in mind is that all of the past tense structures that involve the present participle have some concept of comparing a range of time and a point in time. To further complicate things, this is often implied rather than stated.
So, it isn't typical (and my feeling is that it isn't quite correct) to specify a range of time with "I was waiting." This one simply relates an occurrence to an ongoing activity, without specifying anything about the activity except that it was happening at the time the event occurred. To show a range of time like this, you need to use the pluperfect continuous: "I had been waiting". Consider these:
I was waiting at the station when the train came.
I waited at the station for 3 hours before the train came.
I had been waiting at the station for 3 hours when the train came.
Notice the use of "before" in the second example and "when" in the third. Before has the effect of dissociating the two events, and when has the effect of associating them.