automatic extraction of dtm using grass gis

i have generated dsm using erdas lps tool which is in img format. i want to import this file in grass gis and extract dtm. can you please tell me how to input the data , in which format it should be, what is the algorithm behind the extraction of dtm, what should be the parameters to be given to extract dtm in urban areas ? i tried to import img file and ascii file but it is giving error