Maybe your teacher didn't understand the question since the normal case is for 'iru' to be appended to a transitive verb (the progressive tense) showing an ongoing action or something that is part of a normal routine.
Hon wo yonde imasu. Mainichi shinbun wo yonde imasu.
Otherwise, when it is attached to an intransitive verb, such as Hairu, the explanation gets fuzzier but basically what is being shown is an ongoing state that has consequences visible to the speaker. To ga shimatte ite hairenai desu. (The door is shut and I cannot enter).
Hope that helps at least a little....