The progressive or continuous aspect is used to denote a temporary action or state that began at a previous time and continues into the present time (or other time of reference). It is expressed using a form of the auxiliary verb to be (conjugated appropriately for tense etc.) together with the present participle (-ing form) of the main verb: I am reading; Were you shouting?; He will be sitting over there.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/verb-tense-progressive